A "LAMP stack" is the basic set of applications that run on most web servers.

The name is the combination of the initials of the involved applications:

• Linux - free Operating System (actually Linux is just the kernel of it, but don't bother caring about that)
• Apache HTTP Server - the application delivering content to your web browser
• MySQL - currently the most popular, but by far not the only Open Source database management system
• PHP - scripting language for dynamic web pages (Perl used to be in that place, but PHP became more popular over time)

Because all of these applications are free and Open Source Software, they are the most cost-efficient solution for most web hosting providers!

Alternatives:

Java Web Application Servers or Windows + ASP.NET are also available by web hosting providers, but for more money and with a significatly lesser market share.