A map without data layers is like an artist with a blank canvas. The data layers that you add to your map give it meaning and set the stage for analysis. These data layers come from map services published by ArcGIS Server and, occasionally, other GIS servers. The two main types of layers available from ArcGIS Server are dynamic map service layers and tiled (or cached) map service layers.
Dynamic map service layers reference map services that create a map image on the fly and then return the image to the application. This type of map service may be composed of one or more layers of information. For example, the Demographics map service displayed in the following screenshot is composed of nine different layers representing demographic information at various levels of geography:
While they can take somewhat longer to display in a client application since they must be generated "on the fly", dynamic map service layers are more versatile than tiled map service layers...