A miter saw is a power tool which makes it easy to cut accurate miter joints. The simplest miter joints are those used to hold the pieces of a picture frame together, or the trim around doorways or windows. In a typical miter saw setup, a hinged circular saw is mounted on a saw table in a way that will enable you to make plunge cuts.
Unlike a cutoff saw, or chop saw, both of which will make a cut perpendicular to the saw table and at a ninety degree angle to the long axis of the board, a miter saw can cut at a variable angle relative to the long axis of the board. Most miter saws can also be tilted either side of vertical to produce compound miters with cuts that are angled in two dimensions.
A well-designed miter saw will have a blade brake that quickly stops the blade rotation after the cut is made. It will also have some means of connecting to a woodworker's dust collection system, or to a shopvac. Some saws include a laser guide to mark exactly where the cut will be. A safety shield to protect errant fingers from being mitered along with the wood is always a good idea.
By using a modern miter saw, you will be able to easily produce accurate joints that would be the envy of the craftsmen of yesteryear. In the past, a woodworker would miter a joint with a miter box and a backsaw. Joints made this way are rarely perfect, and a degree of hard-earned skill (not posessed by the weekend woodworkers of today who may not be able to cut straight with a handsaw) is required to make an acceptable joint with this method.