Plesk is a very useful tool, often used by VPS hosting companies. It provides a graphical frontend to the command line that controls the VPS. It is a very useful tool to help novice users get the hang of working in a VPS environment, without having to go through the trouble of learning a myriad of command line options and program names.
Plesk is mainly used to develop web applications, and it serves that purpose well, as it is very flexible in terms of resource management, and it???s also cross-platform and can be run both under Windows and Linux.