A new trend now is to send prospective guests for your wedding a 'Save the Date'
Save the Date is a pre-invitation memo officially announcing your wedding date and lets guests know that they will be invited to the celebration. This can help with guests planning any holidays etc.
They are particularly helpful if you are planning on asking guests to travel abroad for your wedding as it gives them time to save!
Save the Dates should normally be sent out 8-10 months in advance.
If you are travelling abroad for your wedding be sure to include this. At this stage the Save the Dates should only go to the people you intend to invite to the full wedding - saves any confusion later.
They can be as simple as you like (these are not your formal invites - so don't need to match your wedding colours) with main points being your names, the date and location.