Do you not know how to use google (or any other search engine)? You will find more than you need if you actually try looking for it.

Try http://en.wikipedia.org/wiki/Global_variable

Global variables are variables that are declared outside a function, meaning that they can be accessed anywhere in the program by any function. Any variable declared outside of int main() is a global variable. Using global variables is not good programming technique, although it can be done.