Have the following file:

    int foo ()
    {
        return 3;
    }

    int bar()
    {
        return foo();
    }

put your cursor somewhere on the "foo" identifier in the second to last line and type "gD" (without the quotes). It should jump to the definition of foo. It's not perfect but works for much code without crazy preprocessor stuff. By default it's file-local, but you can give vim a list of include files to search as well.