A forged knife is made of a single bar of steel, which is heated and then pounded into shape, often by a specially trained craftsman. The alternative is a stamped knife, in which the blade is "stamped" or cut out from a large sheet of steel, and then the blade is honed and heat-treated for durability. A forged knife is typically heavier by several ounces than a stamped knife.
You can also usually tell a knife is forged because it has a bolster, the wider lip on the end of the blade where it meets the handle. The bolster is intended to balance the blade and to serve as a finger guard. Forged knives also usually have a full or partial tang, which is an extension of the metal blade that extends into the handle. If the handle of a knife has metal rivets, or if you can see the metal along the edge of the handle, that's a good indication that the knife has a tang, which is a sign of quality and durability and helps with balance of the knife.
Forged vs. Stamped Knife: Which should I buy?
The process of making a forged knife is usually more expensive and time- and labor-intensive than making a stamped knife, so it's often pricier. It used to be that a forged knife was better-quality and more comfortable and efficient to use than a stamped knife, but today, you can find high-quality stamped knifes and you can find poor-quality forged knives, so the lines are blurring. It's best to choose a knife based on how it feels in your hand and how comfortable and efficient it is when you're actually doing cutting tests (Fine Cooking magazine says that the best tests to perform on knives include mincing parsley, dicing an onion, slicing a hard winter squash, cutting a carrot into strips and carving a melon). Looking at knives from well-known and well-regarded brands is also a good way to narrow down your decision.