Ura- characters come from the Japanese model called the UraTama. It is similar to the V4.5. The model before that was called the EnTama, and is similar to the V4.
"Ura" generally means the other side / the dark side / the opposite side / the back side. In the case of the UraTama, "Ura" means the inside. The EnTama characters live on the surface of Tamagotchi Planet. UraTama characters live inside Tamagotchi Planet. In the opening scene of your Tamagotchi character's visit to the UraTama Town web page, you see her/him going into Tamagotchi Planet's mouth.
You can get Ura- characters only on the v4.5 (which was sold as the V4plus in Europe.)