The abbreviation MAMP stands for: Macintosh, Apache, Mysql and PHP and it installs as a simple OS X application (drag folder into /Applications). Once you run it it starts up a local Apache and MySQL and shows a simple control panel which you can use to administer them. A full PHP stack is also included (either 4 or 5).
I was using the local Apache and PHP but this is actually much easier to work with, you can change configurations easily, making working with multiple sites much quicker. Although I don't use MySQL at this point (SQLite works for the small sites I am doing) I imagine it's a big help there too.
There is a pro version as well with many more features for managing testing and deployment of larger sites. It also supports external viewing of different sites for folks who want to let others see their progress. Base MAMP is not for serving to the public.
Sure you can do it yourself locally but this makes it totally painless.