Save the dates are very common in the USA, but are increasing in popularity here in the UK. They are usually a postcard consisting of a photo and the date of the wedding on, with further details about the wedding to follow at a later date. It's useful to send them out as soon as you book your wedding venue, i.e. as soon as your date is secure.
There are loads of cute and creative save the date ideas on Pinterest, and here's a few as inspiration to get you started.
If you'd like more save the date inspiration, you can see my graphic design board on Pinterest here.