What does an exhaust tip really do?

I've been hearing about these exhaust tips you can put on cars, do they really do anything except change the sound?  add any horsepower or anything?  What kind of sound does an exhaust tip make once put on? How does it change the stock exhaust?

most tips just look pretty. You can get some that light up or make whistling noises, but all for show

Some have built-in resonators to deaden any unwanted buzzing that you hear when you get on it. If you get a stainless one, get a double-walled one so it doesn't turn colors on you.

no normally tips dont do anything for the horse power. you still have your stock pipe going all the way up to the tip. its like those kids with the hondas and those big fart cans on the back. it isnt doing anything but changing the sound. if you wanted to get more hp from your exhaust my advice would be custom exhaust and headers  designed specfical for your engine. that would give you the best hp for your engine from the exhaust.