Dave pretty much summed it up, but I just want to add tips if you are planning a cruise.
As Dave stated, don't put any valuables in your checked luggage, but rather in your carry on bag. Also, keep any medications in your carry on bag as well, in case your checked luggage gets lost, mishandled, etc. Also, pack what you think you'll wear to dinner on that first night in your carry on bag, as well as one swimsuit, as it takes them awhile to get all the luggage delivered, and if you have the early seating for dinner, chances are you may not have your checked luggage in time.
I can't remember if this was my first or second cruise, but on one cruise, I did see a cart with checked luggage by the elevators. On the smaller ships, they may have 1 or 2 elevators out of service for passengers, and use that for luggage deliveries. Other passengers also saw this, and apparently saw their luggage on the cart, and just grabbed it off the cart. I'm surprised that the cart was unattended first of all, and second of all, that the passengers just grabbed their luggage and didn't wait for it to be delivered. Maybe they've tightened things since then, as this was a few years ago. I can't remember if it's a barcoded system with luggage tags now, but if it is, then the cruise line may think that bag is missing, when in fact it was just grabbed by the passengers.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas