IBM has an article up on their web site -- Virtual Linux: An Overview of Virtualization Methods, Architectures, and Implementations -- that is a very detailed look at Linux virtualization

It's a good read and covers a variety of topics, such as a history of virtualization, types of virtualization and why virtualization is important. The article is nicely summarized by its author:

That last sentence is particularly true and I have mentioned it before: determine what your needs are and work towards that.

A part of the article focuses on types of virtualization -- one thing to note to avoid any confusion is that Virtual Iron offers native virtualization (see this blog post on paravirtualization and native virtualization for more info).