"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.
There are a few differences between the EnTama and the UraTama.
Hardware differences:
If you go to a screen were most of the pixels are activated (such as the when it's sleeping), and hold it up to the light, you can see that the UraTama pixels are dark blue. The pixels on the EnTama are black. If you compare side-by-side, you'll really notice the difference.
The antenna knob is round on an EnTama, and star-shaped on an UraTama. Same with the beads on the strap.
Software differences:
The characters have changed. Different looks and different names.
The "zoku" (groups) have changed, and the GUTS attributes have changed. EnTama has Intelligence, Style, and Kindness. UraTama has Humor, Gorgeous, and Passion GUTS points. Along with this there are new teachers.
Click here for an EnTama character chart.
Click here for an UraTama character chart.
The Cooking Ingredients have changed.
Some of the rare items have changed.
The web site is different. EnTama characters go the TamaTown. UraTama characters go to UraTama Town. Both UraTama and EnTama characters go to the same travel tour locations (Flower Hills, Pachi Forest, etc.).
UraTama can connect to EnTama via infrared. They can exchange gifts (which may get transformed in the process) and they can play games, but they can't mate together.