Hubs are essentially repeaters, operateing on bits.
layer 1 device;
taking an input and retransmits the input on the hub's outgoing ports.

Bridges are layer 2 devices.
using the concept of store-and-forward transmission.
interconnecting networks.

Routers are layer 3 devices.
interconnect physically different networks that support different network protocols.
store-and-forward packets using network-layer addresses. (bridge forwards packets using LAN addresses.)

不知道用中文怎么讲，你将就着看吧。
正好帮我复习一下。^-^