If you are a C# developer then you may keep hearing about all the cool kids from Smalltalk, Ruby, Python, Scala using these crazy things called mixins. You may even be a little jealous, not because you want the feature, but because they have a feature with an awesome name like "mixin". The name is pretty sweet. And in fact, it is fairly self-explanatory since mixins are all about "mixing-in behaviors".
It is actually an Aspect Oriented Programming (AOP) term which is defined by wikipedia as:
A mixin is a class that provides a certain functionality to be inherited by a subclass, but is not meant to stand alone. Inheriting from a mixin is not a form of specialization but is rather a means to collect functionality. A class may inherit most or all of its functionality by inheriting from one or more mixins through multiple inheritance.
No wonder people are confused! That isn't exactly clear. So let's try to clear it up just a tiny bit...