There are three main types of sanders: Belt sanders, Orbit sanders and Random Orbit Sanders.

Belt Sanders:

This might be what you are most familiar with when you imagine an electric sanders. The positives for this sander are that it is good for sanding large areas due to its size. However, the negatives are that it is not well suited for sanding small areas and can be unwieldy to use. Also, changing and loading the sandpaper can be cumbersome.

Orbit Sander:

An Orbit Sander moves or vibrates in a circular motion. Due to this motion it often leaves a circular pattern in the area in which in has sanded. Also, replacing the sanding paper can be cumbersome, as it often involves cutting sand paper to size and using a clamp mechanism to keep it in place. However, of the three types of electric sanders this one is usually the cheapest. For example, the Orbit Sander shown below is only $25 on Amazon, see the following link: SKIL 7292-02 2.0 Amp 1/4 Sheet Palm Sander with Pressure Control

Random Orbit Sander:

A Random Orbit Sander moves or vibrates in a circular motion while also moving back and forth. This can take a little getting use to when using it but also means that it is less like to leave ring marks in the would being sanded. Also, replacing the sanding paper is very easy, and it usually consists of disks which velcro on to the sanding surface. For these reasons the Random Orbit Sander is my favorite sander. The Random Orbit sander I personally use and would recommend is the Bosch ROS10, a picture of this sander is below and it can be found on Amazon here: Bosch ROS10 120 Volt Random Orbit Sander