The two main kinds are studio easels which are fairly large, and portable easels which fit in a small space when folded. Check out some online art suppliers for descriptions and photographs (e.g. Dick Blick).
Folding easels can be used in the studio as well as outside so they are often a good choice for beginners.
An easel isn't always necessary. You can work with your sketchpad on your knee, or with your board resting on the back of a chair, or on the floor. Some people find that having their work on the floor gives them more freedom.