LAMP is a collection of open-source tools that provide full-service web solutions. One of the things that makes LAMP so popular is the wide variety of hosting solutions for it; it's easy to find an inexpensive hosting service that offers these services.

One of the interesting things about taking a LAMP approach is the total flexibility of the solution. Any, or even all, of the particular technologies can be replaced, yet you will still have a LAMP solution. Let's look at the individual pieces (experienced developers, please bear with me):

Linux practically defines the term "open source," and represents the generally free or low-cost operating system behind it. Linux, as we know, is offered in commercial, hardened versions by a number of vendors, such as Red Hat and SuSe. It's often replaced by FreeBSD, (a free descendent of the original UC Berkeley Operating System). Many hosting services run in FreeBSD, and Apple's OS X runs under something very much like FreeBSD. Windows can also host other LAMP technologies, such as Apache, PHP, and MySQL.

Apache is the open-source web server software that actually responds to web requests, and is a powerful, flexible, yet fairly easy-to-manage tool. Most sites are running version 1.3.2 of Apache, though version 2 is growing in popularity. According to Netcraft, nearly 70% of all websites today are being served by Apache, and just over 20% are running Microsoft's Internet Information Server (IIS).

MySQL is generally considered to the database program most widely used for web purposes. It's a fast, easy-to-use, generally standards-compliant SQL database, and a variety of tools (such as phpMyAdmin) and support by popular scripting languages make it popular. The software is in transition now: version 4.1 with some sorely need capabilities (most notably, subqueries) has just been released, and a major new version 5, with significant new powers(stored procedures, triggers, functions) is due for release this summer.

A major issue going forward with regards to MySQL will be the costs (in time and work, not necessarily dollars) of upgrading, and how fast or slow providers include it on their servers. I think there will be a clamor.

Version 5.0 will bring MySQL much closer to Oracle, Sybase, and SQL Server in power, and it might make more inroads into non-web uses. It's hard to measure how many websites use MySQL, as that information isn't necessarily exposed to the public, as OS and server information is. Some sites use PostgreSQL, a generally more powerful SQL database engine instead of MySQL. Both MySQL and PostgreSQL can run under Windows in addition to *nixes, making it easy to mix and match.

And lastly, PHP has become an extremely popular scripting/templating language for web development. The "P" in LAMP can also refer to Perl or Python, both of which are also popular, though a bit harder to use as templating languages. As with MySQL it's difficult to measure exactly how many sites use these languages, although searches of job sites show demand for PHP scripters is strong, and Python looks to be growing.

As with MySQL, the PHP world is undergoing a big change. Version 5 was recently released, requiring some changes to implement old solutions, and adoption by hosting services is still pretty limited. PHP and Perl both run in Windows much the same way they run in *nix.

I should state again that one of the key things about these technologies is the way they mix and match. For example, I have a Windows server at home, and have been working with a client of mine on a FreeBSD server. But once I had them set up, to script them I didn't need to worry about making changes between my test environment on Windows and the production environment on FreeBSD.

I also have components running on my iMac, which was a bit easier to set up that Windows, as it came with Apache and PHP installed.

I agree with assessments that the LAMP open source stack is emerging as a strong challenger to the J2EE and .NET stacks. Given that these technologies are open source and very widely available, I see no reason to drink Microsoft's or Sun's Kool-Aid.