Yep: What Donna said.
When you check in you will secure your account with a credit card or a cash deposit. You then will be issued a ship's card that you will use for all of your purchases and as the key to your cabin.
You will often see the phrase "On Board Credit, or OBC" used on the boards. On Board Credit is an amount of money that has already been put in your account. Cruise lines will do this, on certain sailings and promotions, as a booking incentive. Travel agents can also give you an On Board Credit as a reward for booking with them. These credits will lower your final bill. On Board Credits are not given on all sailings and different agents may not give On Board Credits, especially if the cabin price is very low.
Have a great sailing and just keep reading and asking questions and you'll soon all the "lingo".
Take care,
Mike