Functions which, when called with the right (or the wrong) initial set of parameters, call themselves, causing themselves to call themselves again, and so on ad infinitum. The simplest example would be, in C++:

void foo()
    foo();  //Calls foo, which calls foo, which calls foo...

Given finite computational speeds and call stack sizes, functions which try to recurse forever are generally considered (at least in computer science circles) to be a Bad Thing.

Dekaritae: Well, it's infinitely recursive in theory...