Chef's knife is a loose term for any very large general purpose knife used mainly for chopping and slicing food.
A chef's knife is generally the most used and most versatile knife in the kitchen, used for all but the finest or most specialized tasks. Selecting a chef's knife is therefore very important to comfortable and enjoyable cooking. It should be the largest size which feels comfortable to use (typically 10 inches (254 mm) for reasonably tall cooks, 8 inches (200 mm) for shorter), as a smaller knife will not be able to accomplish as many tasks with ease (chopping large vegetables, halving tortillas, pizzas, and other large foods, and so forth).
Most high quality chef's knives are manufactured from high-gauge, (18/10), stainless steel. The stainless steel blade is either stamped or forged. A forged blade is heavier, usually hand-made, and will keep it's edge making it a popular choice for a professional chef. Stamped knives are cut from sheet metal by a machine, however, this process does not necessarily negate the quality. Forged knives tend to be more expensive, and are seen by some as more reliable. Regardless, a good chef's knife should consist of a single piece of steel, "a full tang". In other words, the steel blade of the knife should extend through the entire length of the wood or plastic handle. A chef's knife containing a full tang is crucial for providing a balance point closer to the hand and a more durable product.
Keeping a knife sharp is very important. A dull knife requires more force from the user, thereby increasing the likelihood of injury. A steel should be used to hone the edge of the blade. Honing with a steel before each use of the knife will keep the edge sharp. Occasionally a sharpening stone or high quality electric sharpener may be needed to bring a severely dulled blade back to life, followed by a few swipes on the steel.
The most important aspect of choosing a knife, however, is how it feels in your hand. The best way to choose a knife is simply to go try the heft of several and see what suits you best.
To purchase a high quality chef's knife, try a local restaurant equipment supplier, or a well known cooking enthusiast retailer. Stay away from the cheap imitations for sale at the grocery or chain discount stores.