Seeing as nobody seems to be able to give you a straight answer....
A 'launcher', or more commonly referred to as a 'home replacement', is basically the thing you're brought to when you hit that home button on the button of the phone. Most have more functionality than the default homes (your Epic has touchwiz, which frankly isn't one of the better ones out there).
They can bring things like say... graphical enhancements.
They can make things move with more fluidity (Launcher Pro), add different transition animations all together (GDE), change the way you swipe from home screens (DxTop and Slidescreen), and so on and so forth.
They add function by adding things like docks to the bottom (ADW Launcher, Launcher Pro, GDE, almost all have them), adding their own special launcher-specific widgets (application dock on GDE, HTC Sense like widgets on Launcher Pro Plus, etc.), and other surface level things.
They can also give you a higher level of customization to use on your home screens to make it more suited for you. Launcher Pro and ADW Launcher allow you to change the highlight colors, the elasticity effect you get after changing screens, the amount of home screens, the default home screen, the theme, the skin of the application docks, what action is performed when pushing the home button, how many icons can fit on-screen, the list goes on and on.
Just for a little sample, I found out how to do this from XDA with Launcher Pro, and this is how I use my phone day to day.
You can do LOADS more with alternative home launchers than you can ever dream of doing with the ones that come stock on the phone.
I say look into getting another one, unless TouchWizz fits you fine as is.