Some recipes call for peeled tomatoes. You'll be at it all day if you just start picking with a paring knife. The skins will slip right off if you follow this method.
Here's one way you can peel a tomato. While a pan of water is being brought to a simmer, remove the core from a tomato using a small knife. Set the tomato into the simmering water for about 30 seconds. Remove it with a slotted spoon or strainer and place in a bowl of ice water. You could also run it under cold tap water to cool it down. After the tomato has cooled slightly, use a paring knife to peel off skin from core to end. If not easy to peel, dip in simmering water a few more moments.