Ethernet is used either half or full duplex. This means that either sending or receiving takes place, or both at the same time.
When Ethernet was created it used a shared medium (coax cable, see EthernetHardware). This meant only one station could send at a time, the others needed to listen. As we've seen before this is half duplex. With the advent of UTP cabling all Ethernet connections basically became point-to-point. Traffic both ways is carried on separate wires, hence sending and receiving can take place at the same time. As we've seen before this is full duplex.
Hubs break the UTP point-to-point model. The send and receive wires from the end stations are connected to the uplink receive and send wires, and among each other. This recreates the old model of a shared medium, which as we've seen above can only be used half duplex.