Node is the generic name for any device connected to a network. Nodes can be individual PCs, servers, printers, fax machines, auxiliary hard drives or another piece of hardware connected to the network device. This device is typically a hub, a switch whose main function is to send and receive signals along the network between the nodes attached to it. A workgroup is the collection of nodes connected to a single hub. This may be several PCs, a printer, fax machine and modem.
Topology is a term that describes the network layout, or how the devices on a network are connected.
The two most common layouts for a network are star and bus layouts. A star layout looks like a star, if sketched out. At the center of the star is a hub, connected to all the nodes in the workgroup. A cable runs directly from the hub to the network adapter card on each PC or device. This layout can work as a single workgroup or with multiple workgroups connected in a larger network.