A socket set contains a number of different types of sockets, ratchets and accessories to cover a wide range of fastener types. They typically come in either metric or SAE sizing, and some may even include an adapter to accommodate both types.
There are also several different sized drive sockets, with smaller ones often being useful for reaching small nuts or bolts in tight places. Metric socket sets are usually available in three drive sizes, ranging from 1/4-inch to 3/8-inch.
The best socket set for you will depend on what sorts of tools you'll be using them with - for instance, if most of your work involves impact wrenches, you'll want to find one that has the sockets you need for Torx, spline and Robertson bolts.
Choosing a good set is not difficult, but there are a few things to bear in mind to ensure you get the right set for your needs.
12-point hex sockets (sometimes known as bi-hex or 'bi-star') are a great choice for general purpose fastener seating, combining the strength of six-point hex sockets with twice the amount of inner faces. A 12-point hex socket is also more flexible than a six-point hex, allowing square-head nuts and bolts to be seated easily.