Deejeying as we know it today arose with the invention of the LP in 1948. Before MP3 and CDs, turntables were the only available medium to store music on. Their influence has been so big that nowadays more and more DJs start using them again. They are usually sorted into two groups: belt driven and direct driven turntables. A direct driven turntable?s motor axle is, at the same time, the turntable's axle. This means that during e.g. scratching, motor sounds might be heard. Their big advantage, however, is their durability. A belt driven turntable?s motor movement on the other hand is transferred onto the turntable itself via a rubber strap. This minimises unwanted vibrations. But their big disadvantage is that when you are externally moving the vinyl, the rubber strap can slip. It is therefore not suited for all types of Deejaying.
So before buying one, really consider what you ultimately want to do with them.