Cashback deals
These deals vary but, as the name suggests, you get cash - in addition to the money you're going to be borrowing. You may use it to pay for moving costs and furniture etc.
Cashback deals are perhaps best seen as a sales technique to get you to take out a mortgage with a particular lender.
It's very rarely a genuine gift and is probably used to tie you in to the mortgage lender - who will eventually more than make their money back.