No map is complete without data to show. Data is added to the map through layers. Layers refer to data sources that include geographic, symbolic, and even tabular data. The ArcGIS API for JavaScript contains a number of modules for interpreting different kinds of common geographic data sources. Some of the layer modules work with data provided by ArcGIS Server, and others can display data from other sources, such as KML, Web Map Service (WMS), and CSV files.
ArcGIS Server provides map layer data through Map Services. Map Services provide data published from an ArcMap map document (.mxd). The visual map data, legend, and underlying data tables are served through ArcGIS Server, and can be consumed through the browser when the layer data source is loaded into the map. Different layer types have different functionalities. We will review those layers in the following sections.