A buy-to-let mortgage is when a buyer purchases a property with the intention of renting it out to tenants. Some landlords do this as part of an investment strategy, but increasingly it is individuals changing from a residential mortgage on their own home, to a buy-to-let mortgage when they move out (perhaps to move in with a partner or during a period of working abroad).
Buy-to-let mortgages are very similar to a mortgage on your home, with fixed rates and trackers available. However, you should expect higher arrangement fees, as well as a different way of assessing whether you are eligible for the mortgage.
Most mortgage lenders link whether you can afford the mortgage to the rent you are charging or propose to charge. Generally speaking, you'll need the rent to be at least 125% of the mortgage payment in order to satisfy your mortgage lender, but you should check carefully before making an application that you will be eligible, as a significant number of lenders have more stringent affordability requirements, especially for higher rate taxpayers.
Remember too that while the lender may consider the mortgage affordable, you need to be happy it is too. Lenders don't factor in the other costs of running a buy-to-let property, such as insurance, agent's fees and other maintenance costs.