Google is your friend. The first match for “mysql routine function procedure” is this: http://dev.mysql.com/doc/refman/5.0/en/stored-routines-syntax.html
A quick summary:
A stored routine is either a procedure or a function.
A procedure is invoked using a CALL statement and can only pass back values using output variables.
A function can be called from inside a statement just like any other function and can return a scalar value.