What in detail exactly is a parite virus Cont

I've spent a lot of time fixing computers cleaning them of virus and adware. Ive encountered Parites but I don't know exactly WHAT they are in detail, what makes them tick. 

Is there anyone who can give a detailed description of what exactly it is? Maybe knowing my enemy more will help me fight it better if god forbid I ever encounter one again. And while you're at it, is there anything else like a parite?