What exactly does a gel overlay do to your nails?

So I am leaving in 3 days on a trip and I want to get my toenails done. However, I don't like the tips of my nails....I cracked them so much against the edge of the couch and whatnot. I have never gotten any of my fingernails or toenails done and I am pretty much new to this. Someone please help me out here and give me some advice...Thanks!!