VMware is a program designed to allow an OS such as Windows and or Linux to load another OS within a virtual window to run that OS on top of the other sort of speak.
Say you have Windows, and you want to run linux too, but don't want to destroy your partitions or repartition you drive to get Linux on your computer. So you load VMware, you can install Linux on it, and run it along side Windows at the same time.
Well, that is the best I can explain it.
Goto
http://www.vmware.com/ to learn more about it from the developers of it.