A map without a legend, also known as a key, is little more than a picture. A legend contains all the information you need to decipher a map's symbols. Legends explain all the meanings of the dots, colors, lines and topographical features on a map and pertinent information like height above sea level. Traditionally appearing on the lower left corner of the map, the legend may be complex or simple, but it's essential.
Direction and Distance
Usually a compass rose will be found within the legend. If it's not inside the legend box, it will be nearby. All maps have a denotation of what direction is north. Usually this is presented in a manner where north is "up" on the map, but that's not always the case. The legend will also hold the measure of scale. Usually presented in both a metric and standard option, the scale will be a bar with subdivisions in it marking different distances. A certain distance on the map translates into a relative distance on the actual terrain. With a ruler or even a marked piece of paper you can use the scale to determine how far it is between two points on a map.
Population centers of note are marked on a map as well. The legend will decipher what the markings mean. Usually a national capital is a bold faced star within a circle. A regional capital is usually just a star. Cities are marked with dots of differing sizes. The legend will explain what the different sized dots mean and how much population they represent.
Borders are outlined on maps by lines of differing thickness and broken in different patterns. Usually the bigger and bolder the line, the more important the border. Some legends will break the line into dotted or dashed lines to show differences such as states or regions or even state parks and counties. Roads often also make up borders and can be shown many different ways. Your legend can tell you if the road you're looking at is an expressway, an interstate or a rural route.
Natural features are usually shown on maps. Sometimes they're shown with different colors sometimes different symbols. The legend will tell you if the green shaded area is forest or swamp or if the bending line is a river or a stream. Everything from terrain to mines to vegetation can be shown as well as man-made features such as buildings and roads.