The Task Manager is a lump of Windows NT that has carried through to the present day, although it became most prominent in Windows XP as the first NT-based Microsoft operating system targeting home users. It is an advanced management tool designed for skilled technical users, but the majority of Windows users are forced to come face-to-face with it on a daily basis.
Task Manager gives, on its first page, a list of those which Windows recognizes to be running programs. It provides a convenient End Task button to kill whichever program is selected. This button tries to ask politely first, but if the program doesn't stop of its own volition after about five seconds or so, or Windows has already identified the program as Not Responding, it will immediately offer to terminate the program immediately.
This is as far as many users will ever get. It is often the only way to halt a crashed program that Windows hasn't already exited for you. It is automatically displayed on Ctrl-Alt-Delete for Windows XP using the Welcome Screen; other operating systems with this version of the Task Manager, or XP using the classic Logon Manager, will bring up a panel of system commands, one of which is to Start Task Manager.
This isn't useful, however, if you don't know what's wrong with your computer. If it mysteriously begins running incredibly slowly because some unknown program is using all your CPU time, it does not immediately provide a way to identify which program is using it. The Processes tab, however, does. This tab shows all running processes (under Windows Vista, processes of the current user, and Administrator escalation is required to show all users) and information about them of varying importance. One of the columns is CPU Usage. This can be used to identify the program using 99% of your CPU; right-click on it in the list, and End Process is one of the options, which will do so after one confrmation dialog box without the grace period offered by End Task on the previous tab. This is the usual way to recover from a high-CPU hang.
More advanced users wanting to figure out why the program went into a death loop can use the same menu to attach the operating system's Just-In-Time Debugger, which will usually pause the program for debugging purposes. The system default debugger, Watson, will just take a memory dump of the program and quit, but alternative programs, such as WinDBG or Visual Studio, can be significantly more practical.
The remaining tabs show various system information, and while they can be fascinating to track, they are not nearly as immediately practical as the previous two.
There's quite a bit of power hiding under the hood of Task Manager, and it's a very useful tool; it's extremely practical that Windows brings it up from the secure attention sequence, as people have long learned it as the "uncrash my computer" keystroke, and it offers the means to do just that. Unfortunately, it is definitely aimed at power users, making the frequency with which novices and the computer-mediocre will face it unfortunate. It is likely to be the techiest thing an average user will encounter in Windows on a regular basis.