Sit down is a common arcade game form factor.
Sit down machines are most often driving or piloting titles. These machine are basically a normal machine with a built in seat that sticks out in front of them. Sit down machines do not have roofs on them (fully enclosed machines with roofs are called cockpits).
Most sit down machines have a large monitor, and a generic "racing" style seat that doesn't move. A few more uncommon titles (such as S.T.U.N. Runner, Hang On, and 88 Hours), are shaped like motorcycles, and the entire machines actually move along with the player. These special machines usually do not last as long as normal titles, due to their moving parts.
All sit down machines have a few things in common. They are big, heavy, and very expensive. A new sit down machine will generally cost as much as a new car, and even used examples can cost $4000 or more (USD). Older titles with smaller screens are usually more reasonably priced though.
These machines are rarely converted into newer titles, despite their high cost of ownership (when they are, it is usually a specific kit designed to upgrade to the newest game in the series, like upgrading Cruisin' USA to Cruisin' World). Most of these machines are expected to remain in service for ten years or more, and they often take up to three years to even turn a profit (which is horrible compared to a bartop or upright machine, which will pay for lf in six months to a year).
When purchasing a sit down machine for home use, you should really give it a full examination, as these machines are usually only sold when they have major problems. That $400 sit down driver could very well need $1000 worth of new driving controls and force feedback mechanisms, and the huge monitors inside some of them are prohibitively expensive to replace.