Mentioned elsewhere on this wiki, a Pathfinder campaign is designed to take characters from level 1 through 20. Although I don’t anticipate us ever getting that far, there will probably be a few levels to be gained as we play.
If you are familiar with roleplaying games of any kind (your WoWs of the world, for starters), you are probably used to the idea of leveling up, i.e. you gain new abilities and powers within the class that you selected when you were creating your character. This is not the only option within most pen and paper role-playing games, and prior to Pathfinder and Dungeons and Dragons 4E, it could be argued that this was not even the default result of gaining enough experience for another level.
In the Dungeons and Dragons-esque games, when you gain enough experience for a level, you can choose to take that level in any class available in the game, not just the one that you chose when you created your character. For example, if you were a first level Cleric, and decided that you also really liked the Sorcerer class, at level two, you could take a level of Sorcerer, thereby becoming a first level Cleric/first level Sorcerer.
If your heart desires something like this, don’t let me stop you. I would appreciate knowing in advance if you have plans like this for your character, but only so that I can be best prepared with the extra supplemental information you’ll need when accessing the abilities of both classes. But, you should be aware of the ways that your character may suffer, gameplay-wise, if you should choose to multi-class.
First, the class you choose when you create your character becomes what is known in Pathfinder as your “favored class.” Every time you gain a level in your favored class, you gain either an additional hit point, or an additional skill point; roughly, a choice of at least a 5% better chance of surviving any blow that might otherwise kill you, or a 5% chance to succeed when attempting to use a particular skill. As might be gathered from this information, you don’t get this bonus when choosing to take a level in any class that isn’t your favored one. Half-elves need not be concerned; they get two favored classes.
Second, each class has its own ability score dependencies. Shortly, characters are largely dependent on their six basic ability scores, from where basically every other relevant number is derived. They are Strength, Constitution, Dexterity, Intelligence, Wisdom and Charisma. For each class, some ability scores are more useful than others; a fighter gets much more mileage out of a high strength score than a Wizard, for example, while a Wizard gets more out of Intelligence. Ability scores are very limited throughout the game, and it is not possible to have them all be really high. Thus, a fighter/wizard will be mathematically inferior to either a fighter or a wizard. I want to stress, though, that it won’t be a devastating difference to anyone that isn’t concerned with maximum efficiency.
Third, for the most part, each class comes with enough of its own inherent abilities that combining multiple classes is likely to make keeping track of everything available to your character a headache. This is particularly true for spell-casting classes, where you’ll have a different selection of possible spells from each class, and possibly a different way of determining how many and which ones you can use. But if you’re up to it, obviously it is my job to help you.
This post became much longer than I intended, so I will wrap up by getting back to the point of the second paragraph above. In earlier versions of Dungeons and Dragons, multiclassing was much more common because it was much more powerful. Classes tended to offer very little individually beyond the first few levels, so taking extra classes provided more options with relatively little drawback. In the Pathfinder system, each class is complete on its own; new stuff all the way to level 20, and at least as powerful, if not in many cases more so, than taking extra classes. But, in case you were wondering, it is perfectly legal to take levels in every available class. The choices are yours, and yours alone. Good luck.