Generally speaking they're a fabric "skin" that's held aloft by a pair of criss-crossing tent poles. These poles are made of a series of hollow metal tubes that are strung along an elastic cord kind of like a necklace. The ends of the tubes have connectors to allow them to be joined into 1 long pole.

You then feed the poles through the designated sleeves on the skin, forming an X. on the 4 corners of the tent there are fasteners that the poles can attach to. Now the key is that the distance between these fasteners is roughly 2/3 the length of the poles. This means in order to attach both ends of the poles to the corners, you bend the poles.

The elastic cord helps with this bend, while the metal tubes help resist it, creating tension in the arc that is formed. These perpendicular arcs then hold up the skin of the tent.