The map author organizes the data in a map into map layers. Layers are like transparencies that can be stacked and viewed at one time, or viewed separately. For example, there might be one layer that contains roads, another that contains cities, and another that contains state boundaries. When you turn on all of the layers, you see a single map with all of the data: roads, cities, and states. To see only the states and cities, you can turn off the roads layer to hide the roads.
The map author can also separate different types of data into separate layers, and even separate different levels of detail into separate layers. Additionally, because the map data is on separate layers, you can easily select the features you want from a specific layer without selecting overlapping features from other layers.
A map can also contain groups of layers, which are called layer groups. With layer groups, you can control all of the layers in the group at one time. You can turn layers and layer groups on or off by selecting or clearing the check box next to each layer or layer group name. You can use the right-click menu to collapse or expand the layers list or refresh individuals layers or the entire map.