Characters are awarded experience points (XP) for defeating monsters and overcoming other challenges. Monsters have a set experience point value (in the monster descriptions).
Each character class has a Prime Attribute listed in the character class description. If this Prime Attribute is 13 or higher, all experience point awards to the character are increased by 5%. Additionally, if the character has a charisma score of 13 or more, experience point awards are increased by 5%. Finally, if the character has a wisdom score of 13+, the character gains a third 5% bonus. It is possible for a character to gain as many as three 5% bonuses to experience point awards.
Sub-classes all have an experience penalty attached. Whenever the GM doles out experience points, a player with a sub-class subtracts that proportion from the total before applying it to the character's XP total.
XP bonuses and penalties are added before experience is totalled. For example, a character with WIS and CHA of 13 (+10%) and the Priest sub-class (-25%) would subtract -15% from any XP they receive.
When your character has accumulated enough experience points to reach a new level, you will roll another hit die and may gain new spells if you’re a Magic-user or Fighter-mage. Your combat skills may also increase. In other words, you’ve become more powerful and can pursue greater challenges!
I've tended, in the past, to be a bit uncomfortable with the idea of equating treasure stolen earned with experience gained, but I've come around to it. Sort of.
I'll go this far: characters can use gold in place of experience points on a one-for-one basis only when paying for training. Characters can use gold for xp only up to a maximum of 50% of the xp required to advance to the next level.
For example, a 6th level Fighter requires 32,000 xp to advance from 6th to 7th level. He or she could spend up to 16,000 gp on training to offset that xp requirement, but no more (or at least, if more is charged by the trainer, the extra is wasted as far as experience value is concerned).
My rationale for this is simple: people in our own world routinely pay vast sums of money to become "qualified" in a field, without any real-world experience whatsoever. Why should fantasy game characters not have the same opportunity? The only difference here is that the PCs have to get their on-the-job experience before they train, not afterwards. Plus, it gives the PCs something to do with their vast caches of swag.
Training to advance in level will always cost a minimum of 10% of the level's experience requirement, regardless of any other considerations. This is additional to any gold spent to offset xp.
Training time is dependent on the ratio of experience gained to gold spent. The base training time is one month, plus one week per 500gp used to offset xp requirements. To continue the example of the Fighter above, he or she would have to spend 1 month plus 32 weeks (16,000 ÷ 500) in training — a total of 9 months to complete training to 7th level.