The ruins at Sukhothai are now a Unesco world heritage site. If you want to read more about it, see the above link.
Sukhothai is considered to be the first capital of Thailand. The kings who reigned from the city controlled most of modern Thailand from 1238 until the last half of the 1400s, when the monarchy moved to a new capital further south at Ayutthaya.
Now the old city is just a scattered collection of ruins, with crumbling brick columns and the remains if temples and large stone Buddhas. There are remnants of city walls, as well. The houses and markets were all built of wood, so it's only the important public buildings that remain.
There is now a new town, New Sukhothai, about 12 kilometers from the ruins, which is where I stayed.