An ArcGIS Online webmap is kind of like a "short cut" in that it is a map that has been created complete with data layers, and basemap. It's just stored in ArcGIS Online. If you define your map as ESRI has done in that particular example, it saves you some coding. That being said, you're not required to set up your map like that. If you look at many of the other samples, the map is created and defined in a more traditional way:var map = new esri.Map("map",{
basemap:"topo",
center:[-122.45,37.75], //long, lat
zoom:13,
sliderStyle:"small"
});
In this case, all you really need is an external internet connection so that you can display the ESRI basemap. All of your data can safely reside within your network. This example shows you the more traditional way of creating a map and then adding your data layers to the map after the map has been created.