A published map is a graphic representation of geographic data. Geographic information on a map is displayed as layers; each layer represents a particular type of feature, such as a stream, lake, political boundary, or wildlife habitat. Layers are listed in the table of contents.
How layers reference data
A layer doesn't store the actual geographic data; instead, it references data stored on a computer. Referencing data in this way makes it possible for the layers to automatically reflect the most up-to-date information in a GIS database.
Layers can reference data locally on the computer, over a network, or from the Internet. A single published map may contain layers that reference data in all these ways. The map author determines how the published map maintains a link to the data.
The table of contents
The table of contents lists all the layers on the map and shows what the features in each layer represent. You can turn each layer on or off by checking and unchecking the check box next to the layer. When the layer is on, it draws on the map. Layers are drawn on the map in the order in which they are listed in the table of contents. Layers listed above other layers in the table of contents will be drawn on top of those layers on the map.
Organizing layers into data frames
Layers in the table of contents can be further organized into data frames or group layers by the map's author. A data frame simply groups, in a separate frame, the layers that are displayed together. Data frames are only present in published maps with two-dimensional content.
Using more than one data frame
A map author might provide multiple data frames to display layers side by side or create insets and overviews. When a map has more than one data frame, one of them is the active data frame. The active data frame is the one with which you're currently working. You can always tell which data frame is active because it's highlighted on the map with a hatched frame and its name is shown in bold text in the table of contents. Of course, if a map has only one data frame, it's always the active one.