LAMP is an acronym: it usually stands for
- Linux, the operating system (well, actually only the kernel - so "Linux" actually refers to "GNU/Linux" - but "GAMP" or "GLAMP" just aren't nice acronyms :-)
- Apache, the web server
- MySQL, the database server
- PHP, the dynamic web page scripting language
However, under P you sometimes find Perl or Python.
With this acronym "LAMP", people can refer to a solution stack of software, which can run (dynamic) web sites. An extremely nice feature of this software stack is that it consists only of open source components, giving you all the benefits of OS; another equally nice feature is that the stack can be realized for free, e.g. using the Debian distribution, or just very cheap, using a commercial distribution like Red Hat or SuSE.