Global replacement in a bunch of files in Vim:

:args *.[ch]
:argdo %s/\<my_foo\>/My_Foo/ge | update

This changes the word "my_foo" to "My_Foo" in all *.c and *.h files.  The "e"
flag is used for the ":substitute" command to avoid an error for files where
"my_foo" isn't used.  ":update" writes the file only if changes were made.

*list-repeat*
:bufdo[!] {cmd}
:windo {cmd}
:tabd[o] {cmd}