What is a basemap layer?
A basemap layer is the reference for your map. A basemap layer typically contains geographic information that provides the cartographic context for your organization’s data. The basemap layer shows common geodata such as roads, buildings, parcels, and terrain. A basemap can be built from multiple layers but is presented as one layer in a web map. Basemap layers are optimized for quick display and cannot be queried or edited.
There are many excellent basemap layers in ArcGIS Online, a repository of professionally prepared map services that are ready to use by anyone. If your application requires common geodata, such as roads, terrain, or topography for a map reference, you will likely find a suitable basemap layer in ArcGIS Online. Using a basemap layer from ArcGIS Online will give your web maps a high-quality cartographic base.
If your organization has special requirements for a basemap layer, you can build a custom basemap layer as an image service or map service from ArcGIS Server. If this is the case, you should do a comprehensive inventory on the available basemap layers to ensure that you won't be doing unnecessary work in building a new basemap layer.
Typically, basemaps are created as tiled layers. Refer to the following documents for more information: