Historically: AMP - Apache, MySql, and PHP
Currently: AMP - Apache-ish, MySQL-like, and Programming (probably PHP/Perl/Python...)
AMP is infrastructure typically used for hosting dynamic, database-driven Web sites.
Historically, AMP was known as LAMP, which stood for Linux, Apache, MySQL, PHP. This was followed by WAMP (for Windows) and MAMP (for Macintosh.) Over time, the OperatingSystem has become irrelevant and the choices of database system, programming language, and Web server have expanded. See http://en.wikipedia.org/wiki/List_of_AMP_packages
The thing common to LAMP, WAMP, MAMP or any *AMP is AMP, which is a (Apache, usually) WebServer, a (MySQL, commonly) DataBase management system, and a (PHP/Perl/Python, often) ProgrammingLanguage.
To learn more about the typical components, see:
Installation of package and components
See: LampPlatform, WampPlatform