Yes, lucid dreaming does in fact exist, and the limits to what you can do in lucid dreams are boundless. The definition is simply a dream where you are aware it is a dream. Once you simply achieve one of those, you are on your way toward learning techniques to use within those dreams, but they all take practice. Once you become more adept at controlling things in lucid dreams, you can pretty much do anything you imagine, but it takes confidence, patience, and desire. You could summon up anyone you want, be anywhere you want, do anything you want. But that is a little ways down the road. The most important thing to learn is how to attain a lucid dream in the first place. You'll learn plenty about that if you stick around here.