Task Manager in Mac OS X?

If you just want to kill hanging apps try pressing cmd+alt+esc. This opens a mini version of activity monitor, which allows you to select and kill any hanging applications.

You want to use Activity Monitor in your /Applications/Utilities folder. This is the equivalent of Windows Task Manager. It will give you a full list of all processes running on your computer at that time, importantly the ones running in the background. However, be careful what you decide to kill because it can make your system temporarily unstable (requiring a reboot) if you don't know what you're doing.

You can right-click on the icon of a running/hanging application in the Dock. If it is not responding, the usual Quit is turned into Force Quit. If the program is running fine, but you want to force quit anyway, hold alt.