Type A (Van Cut-away)
These buses are van drive lines with a school bus body attached to them. The chassis (drive line) is made by Chevrolet, GMC or Ford. These come with diesel or gasoline engines. They are generally the lightest duty bus, but they are the smallest/shortest bus available. They can come as a wide body or narrow body. The wide body has dual wheels and is the same width as a full size bus. A narrow body has a single rear wheel.
Type C (Conventional)
A conventional bus is built on a medium-duty chassis. These are built in two parts. The drive line, frame and chassis are built by one manufacturer. The chassis is then sent to a body manufacturer for completion. The main chassis manufacturers were International (now known as IC), Freightliner, Chevrolet, GMC, Ford. Today there are three main manufacturers. They are IC, Thomas & Bluebird.
These buses come in sizes between 5 rows of passengers and 13 rows. Some modern versions have 14 rows on one side. The 11-12 row is currently the standard on the used market. Anything smaller or larger than this will bring with it a premium price due to supply & demand issues.
Type D (Transit)
A transit style bus is also built on a medium-duty chassis. These are usually built in one part. The chassis and body are built by the same manufacturer. In today's market, you will find International (also known as Ward, AmTran or IC), Thomas & Bluebird. You may still come across a Carpenter or Wayne, but these are getting very rare at schools. These come in two set ups. Some have the engine in the front, next to the driver. The others have the engine in the very rear.
These buses come in sizes between 7 rows of passengers and 15 rows. A 14 & 15 row bus have the same amount of floor space. They just have a little more or less leg room. The 13 row is the most common version currently available on the used market. Expect to pay a premium for a 14/15 row, or anything 10 rows or shorter. The shorter ones are much harder to locate in good condition than the standard lengths.