They are not required.
They are actually a fairly modern invention, and are not required. If you are having a wedding in your hometown and most guests live nearby, you may not need a save-the-date. They come in handy if you are having a wedding during summer or on a holiday weekend.
They should be sent out about 3-6 months prior to your wedding.
Save-the-dates don't serve as an invitation substitute, so they should go out far in advance. Err more towards 6 months for destination weddings that might require greater cost or vacation time.
Be as creative or as simple as you like.
Save-the-dates don't need to refer to your theme or colors. You can send out anything from a simple black and white note to a fabulous magnet of your engagement photo.
Solidify your guest list first.
You should never send someone a save-the-date that won't be receiving an invite. So, make sure you've made your list and checked it twice before sending any save-the-dates.
A save-the-date shouldn't come with a RSVP request.
Guests don't RSVP to save-the-dates. That's for your invitation, so don't freak out if you don't hear from anyone.