When the Onslaught development process began, there were only three tribal lands in the set: Goblin Burrows, Riptide Laboratory, and Wirewood Lodge. All three were rare. After another cycle of lands were cut from the set, R&D decided to create five more tribal lands to reinforce the importance of creature types in the set.
Now each major race has a corresponding land, some of which are rare and some of which are uncommon. Some are more powerful than others, so try them out and see which ones you like.