The LAMP stack referes to a common set of open source frameworks and applications that allow you to develop software without incurring the overhead licensing costs of typical solution frameworks.

LAMP stands for Linux, Apache, MySQL, PHP (Python, Perl).

Linux is a very popular operating system that is growing more popularity all the time.  Many server hosting companies choose to use Linux to avoid the expensive licensing costs that come along with such operating systems like Windows Server 2003.

Apache is by far the most widely deployed HTTP web server.  There are far more site being served by Apache than by Microsoft Internet Information Services and IBM Web Sphere combined.  Just a fun fact, Apache got its name from a bunch of patches being contributed to an open source web server.  When the project came together, they called it a - patchy - web server.

MySQL is the worlds most popular opensource database.  There are many large scale websites that are run off of MySQL databases including Google and NASA.  MySQL is a strong competitor to Microsofts SQL Server and even ORACLE.

PHP is one of the most widely used web scripting languages.  It has a large following and continues to imporve every day.

The LAMP stack is a very powerful combination that can allow you to create enterprise applications without having the overhead of licensing costs.