Kitchen cabinets consist of many different materials; particle board, MDF, plywood, and solid wood are the most common. All of these woods have very different characteristics, so it's important to know what you are buying and to be educated on the different types of materials. Most manufactures still use solid wood (maple, Cherry, oak) for the cabinet face frames and doors. Since it's not economical to use solid wood for the entire cabinet due to availability and cost, manufactures have to find an alternative wood for the cabinet sides and backs. Most manufactures will turn to particle board or plywood to keep costs down and still affordable.
Particle board is commonly used for the sides and backs of cheaper cabinets with a lower quality of construction. Particle board is made up of tiny little wood chips that have been compressed and adhered together with some type of glue. Particle board does not have the strength that plywood does and over time will chip more easily during normal wear and tear. Particle board will also tend to swell up if it sits in water for a period of time. Cheaper particle board is made with a very low quality glue that you would use for home projects, however some particle board is made with an industrial style glue and will hold up much better. One of the pros with particle board is that its easier to mill and form more consistently. A lot of manufactures will use a higher end particle board for the cabinet shelves since the precision is better than plywood.
Plywood is usually preferred by manufactures that want to offer a higher end cabinet and still offer them at an affordable price. Plywood is much more durable than particle board and also much more water resistant. Instead of being made from wood chips, it consists of wood sheets that are compressed together and then veneered. Plywood is also much easier to drill and screw into making the assemble much easier.