A fortified wine from the Douro Valley in Portugal, port is made in a range of styles and qualities. Ruby port is the least expensive while vintage port is the priciest.
When cooking with port, use one that you would be happy to drink.
Once opened, a bottle of port should be drunk with 2-3 days.
Given the expense, it would be a waste to cook with vintage port and other top styles, such as colheita and single-quinta vintage ports. Ruby port and many tawny ports, however, are excellent cooking wines. Use either in stews or casseroles or to de-glaze pans when making quick sauces to go with meat dishes.