3. Switch

A network cable can only have one data packet in it at any instant.

So if two or more computers want to place a data packet on to the network at exactly the same time, then a 'data collision' will take place. 

The network protocol is set up to deal with this. Basically it declares the collided data as unusable and forces the two computers to re-send their data packets at a slightly different time.

This is fine for a lightly loaded network with only a few computers on-line. You will not notice the small delay caused by data collisions.

 But imagine what happens when a hundred PCs are sharing the same network and they are all wanting to send their data packets. This will most likely result in thousands of data collisions per second - each one costing a small amount of time. You will certainly notice the network 'slowing down'.

A switch has a number of ports and it stores the addresses of all devices that are directly or indirectly connected to it on each port. 

As a data packet comes into the switch,  its destination address is examined and a direct connection is made between the two machines.