I am building a custom JavaScript application using the ArcGIS JS API. I have gone through the tutorials and am comfortable with adding widgets, working with dojo etc.. but from a high level perspective I am very confused about layers. I've never done anything at all with ESRI or ArcGIS besides the tutorials listed in the guide, so I feel like I'm missing a fairly major piece of information about how all this stuff is designed. (layers especially, map services too, and probably other things).
Is there some manual or documentation that goes through this stuff. The stuff I've read so far seems to start talking about layers as if we're already supposed to know what they are.
Thanks!