All-Terrain Scout Transport

Or Imperial Scout Walker.
Imperial Ground Vehicle used to move small quantities of Imperial Scout Troopers safely through hazardous terrain, with ease.
Often used to scout terrain, or quell local non-urban riots. Seen on the Forest Moon Endor, when the Rebels stormed the Imperial Outpost. (Return of the Jedi)

Actually, the AT-ST wouldn't be very good at carrying troops, unless they sat on the roof - its a 2-seater (or 1 Wookiee and a sack-ful of Ewoks). Its role would more likely be flank protection for larger vehicles and walkers (as seen in The Empire Strikes Back) and heavy fire support for ground troops (as seen in Return of the Jedi). Scouting is an obvious use, though I personally think it would be a tad too tall to operate with any stealth behind enemy lines. But then, stealth doesn't seem to be the Empire's forte.