Everything else is up to you. You can send them as far in advance of your wedding as you desire, and you do not need to send them to every guest on your list. It is best to send a Save the Date card to every out of town guest, but you probably do not need to send one to your parents or members of the bridal party.
Your Save the Date cards can include as little or as much information as you would like. You can make them simple with just the wedding date and location, or you can include accomodation information.
Since Save the Date cards are sent so far in advance of the formal invitation, they can look pretty much however you want. If you already know which invitations you will be using, you can have your Save the Date cards designed to match. It is not necessary that your invitations and Save the Date cards match, but it is a good idea to have the Save the Date cards reflect the overall tone of your wedding. If your wedding will be formal, your cards should be more formal. If, however, your wedding will be casual or themed, you can have a bit more fun with your Save the Date cards.
With all of the decisions up to you, there's no need to worry about etiquette, so let loose and have some fun with your Save the Date cards!
