Since there seems to be some confusion surrounding them, I'd like to point out how each is defined according to me:

Libraries:
Libraries are portable, adaptable pieces of code that are designed to be included inside your code with little or no effort on your part -- they are designed to minimise the amount of coding that needs to be done by providing a stable engine that provides additional features that were not hard-coded into BYOND itself.

Libraries are designed to be included in projects, not copied and duplicated -- the best designed library will be completely generic and require no editing whatsoever. Deadron's all-encompassing library is one prime example.

If you wish to change the procedures in a library, then according to Object-Oriented programming you can 'override' those procedures. If a library is not properly OOP, then you need to actually physically edit the code itself to change the results. Generally, a good library should be designed so the code itself never needs to be edited.

(Yes, s_admin is a very non-OOP library, but it serves its purpose.)


Demos:
Demos are snippets of code that are designed to demonstrate a working example of some code. They are usually designed to be easy to read, and contain a plethora of comments for people who are relatively inexperienced to discover the syntax of the language.

Demos are not meant to be modified or included in a project -- rather, they are designed to be a reference point for creating your own code.

(I broke this rule with my Alternate Shopkeeper demo.)


Tutorials:
Tutorials are designed to explain a type of coding by granting an easy-to-read reference and/or guided step-by-step learning process of the topic at hand.

They are also not meant to be the basis of any code -- they are designed to teach someone how to create their own system by explaining the processes and nuances involved.