These are two legged versions of AT-AT Walker and are highly maneouverable. Although mainly seen in the Forest of the Moon of Endor, they are seen during the battle on Hoth when the Empire attacks the Rebel base. The vehicles are controlled by two people inside.
Chewbacca along with a group of Ewoks manage to take control of a AT-ST during the Endorean fight. When the rebels are unable to gain access to the field, Han Solo comes up with a plan and uses the communications system inside the AT-ST to trick the Commander inside the base to open up.
Due to their size and maneouverability, they are perfect for places which would be too small for the AT-AT. The AT-ST is seen patrolling through the street of Jedha City in Star Wars - Rogue One film before being blown up by an attack by a break-away group of the Rebel Alliance.
An AT-ST makes a brief appearance during the battle on Mimban where Han Solo has been stationed after being thrown out the Imperial Navy for not following orders. The planet is also where Han Solo begins his smuggling career after meeting Tobias Beckett who is there to steal equipment from the Imperials.