Save the date cards are simple little cards telling your friends and loved ones that your wedding date has been set. It's a way to give them notice in case they need to plan ahead to make arrangements for time off work or saving for travel expenses. It's like a formal heads-up that you hope they'll be attending your wedding.
Many brides who have family that live nearby and are planning smaller local weddings save the postage and forgo the save the date cards, which is totally acceptable. Nobody is going to chastising you for poor wedding etiquette if you don't send them out. However, if you're planning a destination wedding or have a number of guest who will have to travel to make it to the church on time, you might want to consider sending them out.
Often save the date cards are offered for purchase wherever wedding invitations are sold, but that doesn't mean you have to go that formal. If you like to get creative or your wedding will have a particular theme or be taking place at an interesting location, you can always make your own save the date cards or crafty reminders. For some memorable ideas your guests will enjoy, check out Save the Date Ideas: 101 Fun and Creative Save the Date Ideas for Weddings by Sylvia St. John. She includes all kinds of interesting save the date tips and concepts that will definitely get the attention of the recipients, from who NOT to send save the dates to to sending coconuts for a wedding in Hawaii.
Giving your friends and loved ones extra advance notice of your nuptials helps them prepare for the big day, too. Sending a save the date card or craft will allow them more time to plan for your big day so they can be there with wedding bells on.