Collision (Carrier Sense Multiple Access/ Collision Detection) CSMA/CD

Collision is the effect of two devices sending transmissions simultaneously in Ethernet. When they meet on the physical media, the signals from each device collide and damaged.

Collision domain

Collision domain is the group of devices that share same collision effects over the Ethernet network.

CSMA/CD

It is a mechanism of removing collision from network. When two or more nodes simultaneously sense the wire and found no signals, and each device places it’s signal on the wire. These signals would be collide in wire and a collision will occur.
Before placing any signal in wire, NIC (Network Interface Card) examines wire for any existing signal. This method is known as CS (Carrier Sense).
If two NICs sense wire on exactly same time and see no signal then both will place their signals in wire. This is known as MA (Multiple Access).

If the NICs see a collision for their transmitted signals, they have to resend the signals. In this situation, each NIC that was transmitting a frame when a collision occurred creates a special signal, called a jam signal, on the wire, waits a small random time period, and examine the wire again. If there is no signal in the wire, NIC will retransmit its original signals again.
The more devices you place on a segment, the more likely you are going to experience the collisions. More devices means more random time interval, creating even more collisions, gradually slowing down a device’s access when trying to transmit the data.
If a 100Mbps HUB has 10 ports, it means each port will effectively get only 10 Mbps (one tenth of total bandwidth.) Situation can be more worse if we connect this HUB with another HUB on uplink port. Each new node will decrease the available bandwidth for other nodes in network.
Bridge solves these (bandwidth and collision) major issues of HUB.