VMWare Server / Player / Workstation is a very powerful tool allowing one to install several virtual operating systems on one computer. This is not all you get, as you have the ability to run all these OS’es simultanously without need to do anything with your real physical computer. On the other hand this allows to separate different tasks on different machines and prevent software incompatibilities and cross-platform issues.
VMWare Player / Workstation: Good for those users who only need another OS on their computer (Windows on a Mac or Linux or viceversa). It’s much simpler to setup and the workstation is a payed version which allows you to exactly determine sharing the resources on your computer (hard-disk, RAM, Processor). While Player only shares these on fair basis.
VMWare Server: As the name says it out, it suits best for servers (e.g. I used this feature to have a Linux OS on a Windows Server 2008 for logging of a process on a Motoman robot).