A product photographer is a more specific type of commercial photographer. He or she is specially trained to capture a product - any product - in it's best light on camera. Whether it be a shoe, a toothbrush, or a highly specialized esoteric piece of equipment, it is the product photographers job to know how to sell the product - just by looking at the picture.
Because of the range of products, there is seemingly always enough work for a product photographer. However, it's important that the photographer knows how to keep the client happy. This is not a high-art form of photography and self-serving photographers looking to create their own masterpieces need not apply. A product photographer is hired for one and only one purpose: make the product shine.
If you're hiring a product photographer, or are a product photographer, it's important to have a contract in place dictating how many shots youre taking, who will own the shots upon shoot wrap, and how the shots are going to be used. It is perfectly acceptable for a product photographer to require more money from a company if the shot is going to be the new slogan, ad, etc that is to be featured on all future marketing materials.
As with most commercial photography, lighting is one of the most essential aspects in product photography. Light tents are a highly utilized piece of equipment in product photography. It's also important that the product photographer has set an appropriate amount of distance between himself and the product so that it is shown in it's best light.
Product photography also may involve shooting moving pictures, or film of a product. While this is probably going to be on the shoulders of a separate photographer, you may be asked to provide assistance.