In the US, a grand refers to any amount of money in the thousands (and some hundreds). It is also used abroad where they have a different currency system, for example five large means 5000. (In the UK it would be a quid, but that's not really a word anymore).
The word "grand" derives from the French term grande, meaning big or great. It is also related to the Spanish word grano, or grain, which is the equivalent of $1. A buck is another word for a dollar, but it dates back to colonial times when deerskins were often traded for goods.