The physical topology of any network is made up of the physical devices that connect the elements or nodes of the network together. Ethernet cables, hubs, routers, computers and air waves that the wireless signal passes through make up the physical topology of a network.
The logical topology is the structure that has been programmed into the intelligent devices to control what individual logical nodes have access to in the network. The logical topology can encompass the entire physical topology or only a portion of it.
A Simple example - three servers - two workstations two printers and a scanner. All devices connected to each other over a physical topology. But logically not all workstations have access to all servers, or other peripheral devices because they have been separated into work groups that cannot see each other. You could have any number of logical typologies that would run over the same physical topology.
I hope that helps.