That's the 'launcher' of the system. It's always running and it is the 'gateway' to everything in your phone. You can use a 3rd party launcher to replace TouchWiz and have something else run instead, but you always need a launcher running.
TouchWiz, however, is not only the launcher as it also runs some parts of your system (that's Samsung's development), so unless you root your phone, you can't uninstall or freeze it, just have it 'laying dormant' if you use a different launcher. And if you root your phone, uninstalling TouchWiz will pretty much screw SOME things in your phone even if you don't use it as your main launcher. Also, if you get rid of it and still manage to keep your phone usable, should anything happen to your 3rd party launcher you wouldn't be able to do anything with your phone once it boots.