There are a number of types of exercise bench. Some are designed for very specific types of exercises, while others allow you to perform a variety of exercises.
The Weight Bench
The weight bench is a padded exercise bench that is mostly used for weight lifting exercises. Some of these are flat, non-adjustable padded planks while others are hinged so the upper body portion can be elevated for performing different types of exercises. Every weight bench has two arms at shoulder level for supporting the weight bar. Some weight benches also have accessories and attachments for weight lifting with your legs and fly type exercises.
Flat Exercise Bench
A flat exercise bench is used to elevate your body off the floor for certain types of exercises, such as sit ups, crunches and reverse crunches. These benches are very similar to weight benches. The main difference is there are no arms to support a weight bar. The padding on this type of exercise bench is also much thicker than normal weight bench padding.
Foldable Exercise Benches
Another style of exercise bench that is very handy for those without much room. These benches fold so they can be stored in a closet or under a bed. This type of bench comes as either a weight bench or an exercise bench. Sit ups, crunches and reverse crunches are examples of exercises that can be performed on this type of exercise bench.
Ab benches are designed for nothing more than working your abs. These benches have a small padded seat on a frame shaped like an upside down letter "U." On the support legs are padded posts used to lock your feet in place for doing ab exercises like crunches and sit ups.
Incline benches are usually used for working your abs. Normally, there are two padded knee rests at the top of the incline and two padded posts to lock your feet under. The incline adds intensity to your abdominal workouts. The knee supports can also be used to hold onto for leg lift type exercises.
Combination Exercise Benches
Combination benches can be configured to be used for a variety of exercises. This type of bench will have removable weight bar support arms, padded knee rests and foot lock posts. Another feature of this type of bench is that they can be configured as either flat benches or incline benches. Ab crunches, flies, bench presses, reverse crunches and leg lifts are all examples of the types of exercises that can be performed on combination exercise benches.
Back Extension Benches/Racks
These are benches that are designed solely for back extension exercises, which are also known as reverse crunches. This type of bench is usually inclined, has only a small padded bench and two padded posts to lock your feet under.
Listed above are only a sampling of the various types of exercise benches available. Each of the types also come in a variety of styles and designs which facilitate the exercises they are designed for. Most gyms will have a variety of these available, except for the folding type which are aimed at the consumer market.