A function is a language element which performs an operation and, possibly, returns a
string. As an example, function inputfile() returns the
name of Cows' input file.
Cows doesn't allow to define your own functions (at least not in this version); the closest thing to a function call is the parameterized inclusion (Chapter 17) but you can't really encapsulate code since variables' scope is global to the whole page.
This manual can be downloaded from http://www.g-cows.org/.