Basically, Fesarius got it exactly right. Skill Groups a re just a convenient way of organizing similar skills. Otherwise they follow the same rules for skills and specialties.
In the case of Knowledge skills, which would seem to allow a character to know information he couldn't possibly know, you could assume that their knowledge of other places and things allows them to make an educated guess as to how things are in a new place. To use you example, Tar, someone with Knowledge: Specific World (Earth) would get a +2 bonus when remembering facts about Earth and just their normal level when remembering things about other worlds. If the character is on a newly encountered world, he could make a Specific World check to try to figure out the similarities between this wolrd and those he knows.
Hope that helps.
Former Decipher RPG Net Rep
"Doug, at the keyboard, his fingers bleeding" (with thanks to Moriarti)
In D&D3E, Abyssal is not the language of evil vacuum cleaners.