(shouldn't this go in Rules?)
Class Skills are easier to purchase than Cross-Class skills. If a Skill is Class, each Skill Point you spend on it gets you one Skill Rank. If a Skill is Cross-Class, each Skill Point you spend on it gets you
one half of a Skill Rank. Some Skills, called Exclusive Skills, cannot be purchased at all unless they are Class Skills. A character can have no more Skill Ranks in a Class Skill than their level +3, or half that in a Cross-Class Skill.
For instance, my character is Jo the Barbarian. He is an Elf with an Intelligence of 10, so gets 16 Skill Points at 1st level. I want him to be good at tracking, scaring people, jumping around, and basket weaving theory. I put 4 points each into Wilderness Lore, Intimidate, Jump, and Knowledge (Basket Weaving). The first three are all class skills, so I have 4 ranks in each of them. For Barbarians, all Knowledge Skills are Cross-Class, so I only my 4 points only buy me 2 ranks of that.
Hope that helps.

--Jeff