There are three options for something to sit on during meditation.

A chair (good if you're starting out or have back problems)

Chairs are great for if you're just starting to meditate, or if you have back problems and find sitting on a meditation cushion uncomfortable. If you're new to meditation, I recommend using a chair the first few times instead of going out and buying a meditation cushion. Once you routinize meditation and become more comfortable with it, then I would recommend purchasing a meditation cushion; using a chair at first will help you ease your way into practice.

A meditation cushion (most common)

A meditation cushion (named a "zafu", pictured right) is the most popular thing people sit on during meditation. The great thing about a meditation cushion is it is easiest to sit in an upright position when you're on one, which improves your alertness and the quality of your meditation (and hence, how productive your sit is). With a chair or a meditation bench, you may be tempted to slump, which can cause you to lose focus.

A meditation bench (more comfortable than a cushion)

If you're taller or find a meditation cushion too uncomfortable, it's worth giving a meditation bench a shot. It will still force you to sit upright, and you won't have the urge to slump as much as you do on a chair. Meditation benches also absorb a lot of the weight you would have otherwise applied to your legs, which makes meditation much more comfortable.

Zafu image by Roberto Poveda.