
Remco Sommeling |

If a skill is on any of your classes skill lists it is considered a class skill, which means you get a +3 bonus if you have any skill ranks in it.
You are free to buy skills on a 1 for 1 basis even if it is not a class skill though. Sorry seems I do not have the book to give you the actual page, I assume it is in the chapter describing skills.

Sylvanite |

Each level, your character gains a number of skill ranks dependent upon your class plus your Intelligence modifier. Investing a rank in a skill represents a measure of training in that skill. You can never have more ranks in a skill than your total number of Hit Dice. In addition, each class has a number of favored skills, called class skills. It is easier for your character to become more proficient in these skills, as they represent part of his professional training and constant practice. You gain a +3 bonus on all class skills that you put ranks into. If you have more than one class and both grant you a class skill bonus, these bonuses do not stack.
The number of skill ranks you gain when taking a level in one of the base classes is shown on Table: Skill Ranks. Humans gain 1 additional skill rank per class level. Characters who take a level in a favored class have the option of gaining 1 additional skill rank or an additional hit point . If you select a level in a new class, all of its class skills are automatically added to your list of class skills, and you gain a +3 bonus on these skills if you have ranks in them.
From the PRD official document.
Essentially, you can put skills in pretty much whatever you want, but you only get a +3 bonus to the skill if it is a class skill. However, you can keep increasing your list of class skills by gaining levels in new classes. That said, Pathfinder, as opposed to 3.5, really has capstones and benefits to staying single classed that you will miss out on.

Elton |

** spoiler omitted **
From the PRD official document.
Essentially, you can put skills in pretty much whatever you want, but you only get a +3 bonus to the skill if it is a class skill. However, you can keep increasing your list of class skills by gaining levels in new classes. That said, Pathfinder, as opposed to 3.5, really has capstones and benefits to staying single classed that you will miss out on.
Thank you for your prompt answers. I was also looking for a page number.