When it comes to cooking, this is a very useful bit of information to know.
Bring a large pot of water to a boil. With a very sharp knife, cut an “X” in the skin on the top of the tomatoes. Plunge into the boiling water for 30 seconds & up to a minute, just until you see the skins peel back and loosen a bit. Remove to a colander and run under cold water for a couple of minutes. Let cool, peel with a paring knife. Cut tomatoes in half horizontally. Gently squeeze out the seeds from the tomato. Proceed to use as required in your recipe.
An alternate method, that I discovered recently, comes from Marcella Hazan, is especially useful for making a gorgeous, juicy in-season tomato salad, salsa or uncooked tomato pasta sauce. Holding a sharp swivel-bladed peeler perpendicular to the tomato, work in a saw tooth motion around the circumference until the tomato is peeled. If the tomato is especially tight skinned, make a slit in the skin first to get started.