The first and most important point I would like to make is that character roles tend to revolve around 4 major archetypes which I will be discussing in depth. These archetypes are in no way rigid templates for character roles, but are simply guidelines based on the common goals of players in a multiple character group RPG setting (such as an MMO or an RPG with a 'party'). I will also be using common terminology for the sake of familiarity, if you ever played an MMO you might recognize these.
The four archetypes I'm referring to are 'Tank', 'Damage Per Second', 'Crowd Control', and 'Support'. I'm sure many of you are already familiar with this terminology in MMOs, and if so you already have some knowledge of what these roles are.
Glorified meat shields
Lets begin with The Tank. As the name suggests it's a character built like a tank: Something that can take a lot of punishment. The main role of a Tank character is to hold the attention of enemies (mobs) in order to protect the rest of the team from being attacked. They push the group forward towards their goal and are usually in the front lines. Tanks behave as the outer protective shell that protects the inner workings of your party so they can properly function without the worry of being overwhelmed.
This role is usually pulled off properly in most games, it is typically defined by a stout warrior type character that uses a sword and perhaps a shield and wears heavy armor. The abilities of a successful tank would be high defense (or at least a way to prevent damage to themselves) along with decent damage output and 'taunting' skills to be able to hold the attention of enemies and to make sure you're in the target of their aggression instead of your teammates. Tanks can also utilize skills that would prevent targets from taking offensive maneuvers against the party, such as interrupting spell casting or 'stunning' enemies.
Common subtypes are characters that prefer to boost their defense and/ or HP so they can survive, usually depicted as a 'Soldier' or 'Warrior', and if they use minor self healing abilities they might be classified as a 'Paladin'. A character might also improve their offensive capabilities instead of their defensive, using offensive tactics to put pressure on enemies and holding their attention. These 'Barbarians' or 'Beserker' characters can maintain more enemy aggression but may take more damage as a result of trading defense for offense, it is common to see these subtypes to become stronger the more damage they take. A less common type of tank is a character who would rely on dodging/ parrying/ blocking damage, while somewhat less unreliable than their counterparts still achieves the same goals. A tank wouldn't even have to be a stereotypical melee soldier either, consider a magician with strong defensive spells and magic that can draw in enemies, a character like that can also act just as good as any of the examples mentioned above.
Can you say 'Backstab'?
Next on the list is DPS (Damage Per Second), as it implies it represents a character that is built to do the maximum amount of damage per second. This role's purpose is to provide the group's main source of offensive damage and to defeat key targets in an enemy group as quickly as possible. Possibly the most straight forward archetype in any game, and also the most common. DPS characters can vary in the method of how they achieve their goal, and it is common to see it overlap with several different roles and cause imbalance. DPS characters are the heavy artillery of the group, they are the ones who get the kills. Typical DPS classes are represented as agile and cunning and use light weapons and armor like knives or short swords, or ranged weapons such as bows. The abilities of a DPS would be ones that increase their overall offensive capabilities or even weaken the opposition so they may quickly dispatch their targets. a DPS character might also have methods of self preservation such as avoiding enemy aggression or being able to escape tight situations.
DPS subtypes usually are split into melee and ranged types, the former commonly using small weapons such as knives and represented as a 'Thief' or 'Assassin', and the latter being one that uses a bow or even a gun (and could even have some sort of NPC 'pet' that fights with them) such as a 'Ranger' or 'Archer'. While weak yet fast attacks are common in DPS characters who prefer to dice up an opponent with several low damaging but rapid fired attacks, Slow but heavy attacks can work just the same, as long as the character's average damage per second is high and can be maintained long enough to efficiently kill a target. DoT (damage over time) abilities such as 'poisons' can also aid a DPS character, or even be the sole source of their damage, so even a magic user character can be a successful DPS.
A rather sexy example
The fourth archetype is CC (Crowd Control), which is a rather evasive role to describe. While commonly represented as the magic wielding class, a CC character's main purpose is not to kill targets (that's the DPS' job) but instead to weaken them. This goal can be obtained in a large verity of ways: You could weaken an enemy's status directly, use large AoE (Area of Effect) or multi-targetting abilities that deal damage to a large area or several targets at once, or even deny enemies from using abilities or slowing their movements. Possibly the most explored role there is, there have been several adaptations which has lead to some confusion of what the role's true purpose is. The CC's role is to weaken or even outright halt an enemy's approach to their own goals by holding dominance over a battlefield, thus controlling the crowd.
Subtypes for this class come in many flavors... But almost always it is represented by some sort of spell-casting class. However, this role has as much verity of representations as any other class, and I implore developers to be more creative and to try not to stick with the linear magic type character and possibly try to be more creative. The most common subtype is a 'Nuker', a class that deals heavy damage in a large area or over several individual targets, a typical "kill it with fire" mage character is a good example. The next common subtype is a character that weakens an enemy directly, usually by decreasing their stats directly and/ or preventing them from taking actions, usually coupled with DoT abilities (like poisons or life-draining spells), commonly defined as a "Necromancer" or "Illusionist". Like I said before it doesn't have to be a magic-type class, it could use melee to achieve it's goals such as a "Saboteur" class that poisons enemies and disables limbs, or an "Engineer" that constructs explosive devices to damage enemies in an area. I personally think this role has a lot of unused potential.
Finally there is the Support, the most currently underrated role. Most commonly the healer, making sure the rest of the team doesn't die, and typically in most MMORPGs it doesn't get any deeper than that. While a support character doesn't always mean restoring hit points, it can also mean applying buffs on teammates or providing backup. A support's main goal is just that... support! It backs up the rest of the team by either preserving them or making them stronger and more efficient and even fighting along side their allies. Possibly the most important role, it is also the least popular and most misunderstood.
Most games stick the support role into a weak spell-caster mold, meaning they have little to no offensive or defensive capabilities whatsoever. Whatever little offense they do have ends up being magic, meaning it usually uses up the same spell points they use for healing, so in a group their offense becomes useless as not only do they have to save their points for healing their offensive abilities are much too weak to make much of a difference. Not only this, but because support characters are the prime targets of enemies due to their importance, they have no way to defend themselves due to a lack of defense or offense (or both). This subtype is sadly the most common and the most broken, if all a character can do is heal then it's not very exiting to play (I personally call these types "healbots"). A much better example would be characters who are balanced in all fields, but excels in support abilities such as healing and buffs. There is also the subtype devoted more to providing strengthening buffs, such as a "bard". This Archetype is possibly the most versatile as support abilities can be coupled with any other role, and I believe that it has the most unseen potential out of all the 4, even more than crowd control.
There are also some common mistakes some MMOs have with strict character class systems that I want to point out:
The most common is grouping CC and Support into one class, usually splitting off into their own roles after a "class advancement". This makes the Support role into a defenseless spell caster who's only purpose is to heal (Like the "Healbots" I mentioned before). Even if the classes are separate most games still portray the Support class as a squishy wizard, which can be fine if given enough depth and doesn't take away from the overall offense of the party.
Another mistake is splitting a DPS class into ranged and melee classes. While this is fine as long as the other 3 roles are given 2 classes of their own, but this usually isn't the case. It will usually end up with DPS having more attention than other roles.
A less common but still prominent example is fusing Tank and DPS into one superior melee class, this is usually coupled with ranged DPS given it's own class. Commonly it will let players split off to either DPS or Tank roles with the class, but it has the same effect as the above example.
I find that the most common example is all three combined (tank/DPS, ranged DPS, melee DPS, CC/Support). The most notorious example being the old Maplestory class setup, before they went overboard...
This has become so common I started to refer to it as the "Korea Syndrome", as Korea being the biggest producer of MMOs and this template being the prominent example in their character development systems. While newer games are deviating from this, the older ones stick to it like glue.
If you keep these major archetypes in mind when developing an RPG character system, it can make balancing much easier if you apply equal attention to all 4 fields. Of course these are not rigid molds, they can be played with and mixed to create an endless amount of potential characters, and creativity is always encouraged.