The Save the Date is an informal card or postcard that has the date and the city of your wedding. This is done as a courtesy so that guests know when your wedding, that way if they need to travel from out of town or take time off work or if they are just generally busy, they will know far enough in advance to schedule it.

The formal invitation will have more information like the time, addresses, parent's names if you are doing that, information about hotel blocks and travel (though, this stuff isn't necessary anymore because people just put it on their website), and an RSVP card (also not necessary as people do electronic RSVPs these days).

The thing about wedding stationary is that these days you can do as much or as little as you want and can afford. So, if you are on a tight budget, you can do the whole thing electronically (saving on printing costs and postage costs, which really add up). If you have a large budget for invitations, you can do paper EVERYTHING. People often end up doing a mix these days because the paper things make very nice keepsakes for the bride/groom and their families, but you can save a lot by choosing to keep some aspects electronic (like RSVP, hotel information, maps, etc.)