VirtualBox is a free software product for creating virtual computers. It installs on the existing operating system as a program. The tool is supported by such virtual operating systems like Windows XP and later Windows versions, Linux, Mac OS X, Solaris, OpenSolaris. Inside VirtualBox one creates virtual computers and installs operating systems on them.
VMware Workstation is a commercial tool used for simultaneous running and controlling several operating systems on one computer. The tool emulates separate machines on which one can install operating systems, and various programs.