Tamriel unlimited is just the base game. Some editions of TU come with the explorer pack, allowing you to play any race in any alliance. (Which is less pertinent since One Tamriel, since you can go to any alliance as soon as you roll a character, although you still need it if you want to PVP a specific race outside their home alliance). If the game includes the explorer pack dlc, it has a sticker on the front.

The Imperial edition gives you access to the imperial race and a special horse. This edition is cheap because that's not a great bonus...imperials are very strong, but most players prefer the other races. This edition also does NOT include any dlc.

You might consider the Gold Edition, which includes the base game and four DLC areas: imperial city, wrothgar, thieves guild and dark brotherhood. The cost of buying these separately is minimum 5500 crowns right now ($50ish). So this is the best deal if buying the disc to play the game.