Global route is the first step in routing the design, kind of a planning stage. The design is broken up into boxes called gcells and the general plan of what wires will go through which gcells in order to connect up all the nets is determined. This does not result in routed wires you can actually see. The detail route is exactly what it sounds like, the actual routing of the wires. This step will result in wires that you can see. The most common way to route a design is to do both of these steps in one command, globalDetailRoute.
Hope that helps,