include_once as well.
So your question should be...
The answer to 1 is described here.
The require() function is identical to include(), except that it handles errors differently. If an error occurs, the include() function generates a warning, but the script will continue execution. The require() generates a fatal error, and the script will stop.
The answer to 2 can be found here.
The require_once() statement is identical to require() except PHP will check if the file has already been included, and if so, not include (require) it again.