What makes MAMP interesting is they bundle together current, up-to-date, versions of Apache, MySQL and PHP. Rather than the Apache 1.30 which Apple distributes, the MAMP includes Apache 2. Rather than PHP 4.1.x which Apple distributes, with very few of the interesting extensions, the MAMP includes PHP 5.x, bundles many of the useful extensions, and bundles the eAccelerator.
Further, the MAMP includes a nice GUI for starting, stopping and configuring the MAMP environment.
As they say, getting the thing going is a simple download, dragging the MAMP folder to /Applications, and double clicking the icon to start the server.