Some (probely simpel) questions about modding

Quandrastorm

While working on the Evolution mod, i ran into a few question, i manage to get things working with not touching certain things but id still like to know how to handle them ;D

 

What to do with networkVars? are they kept synchronised between all clients & server or do they have a different purpose. and is it easy to just add things there? or do i need to register new variables first?

 

What is meant by all the Mixin's? i see that term is used quite a lot but i dont know what it is seposed to do