this should help.
"A task manager is a program used to provide information about the processes and programs running on a computer, as well as the general status of the computer. It can also be used to terminate processes and programs, as well as change the processes' priority."
What is the Task Manager you might ask? Well in Windows XP the button combo CTRL+ALT+Delete opens this up. In Vista and Windows 7 You get options such as " Lock this User,Switch User,Log Off,Change a Password and Start Task Manager.
Regardless CTRL+ALT+Delete is usually used to access the task manager for the above description. Example : When a program isn't responding or is frozen you can access the Task Manager (Ctrl+alt+delete) and find the program or process that is frozen and end it manually if It won't respond.
Hope this helps as well as clears up what it's used for.