The way the word “level” (as in “character level” and “class level”) is used in the Worlds of Adventure campaign is a bit different from how it’s used in Pathfinder and similar games.
In Pathfinder, a character’s “character level” or just “level” is the sum of all of their class levels: a 2nd-level fighter / 1st-level thief is a 3rd-level character. In Worlds of Adventure, this is not necessarily the case.
In Pathfinder, “hit dice” is (for player characters) the same number as “character level”. In Worlds of Adventure, this is still true.
In Pathfinder, you get one hit die from every class level you have. In Worlds of Adventure, this is not necessarily the case.
Here’s how it all works in Worlds of Adventure:
You pay gold to buy XP, and spend that XP to raise your level in classes of your choice. (That is, in the terms of the campaign world, you’re paying the Argos Trade Consortium for training; and choosing what training—in which class, in which skills—you wish to receive, is represented as “spending XP”.)
How much XP you have spent to improve your character—summed across all your class levels in all the classes you have—determines your character level. Table: Character Level tells you what your character level is, based on how much total XP you have spent (note: spent, not bought/“earned”):
You can never have more hit dice than your character level (at least, not permanently; temporary effects, such as a bard’s inspire greatness ability, can raise your hit dice beyond this limit). Your base save bonuses are also limited by your character level; none of the base bonuses for any of your saving throws can ever be higher than the maximum base save bonus listed for your level, on the table above. (Of course, your ability score modifiers, magic bonuses, and other modifiers can increase your total save bonuses without limit.)
(For rules on epic characters in the Worlds of Adventure campaign, see Beyond 20th Level.)