Hub

Hub is used to connect multiple computers in a single Network.  It’s a Layer 1 (Physical Layer) device which simply Broadcasts the receiving signal.

Example: 

Here Sys A sends a data to Sys D, when hub receives the signal from Sys A it forwards the signal via all the other ports. Now all the Computers connected in Hub except Sys A will receive the data but only Sys D will pick it. Hub is commonly known as a stupid Device, because it simply broadcasts the signal all over the network that makes the bandwidth down and causes Collision.