In Kingdom of Loathing, your character has six stats to worry about:

You have three primary statistics, Muscle, Mysticality, and Moxie.

Muscle
Muscle is a measure of your physical strength. It is used to determine your chances of scoring a hit in combat, and to determine your level if you are a Seal Clubber or a Turtle Tamer.

Mysticality
Mysticality reflects your attunedness with all things Magical and Mystical. It is used to calculate your maximum MP, and determines the effectiveness of certain skills. It determines level for Pastamancers and Saucerors.

Moxie
Moxie is a measure of your panache, your chutzpah, your cojones. It determines your chances of dodging enemy attacks in combat, and determines your level if you are a Disco Bandit or an Accordion Thief.

As you adventure, each of your primary stats will gain the equivalent of "experience points." When you see, for instance, "You gain 5 Strongness," it means that your Muscle is being exercised. When you accumulate enough of these points, your primary stats will increase, and when your primary stats get high enough, your level will increase.

There are three other statistics to your KoL character:

HP
For those of you who have never played a computer game before, HP stands for Hit Points. It's a measure of how much physical damage your character can take.

MP
For Seal Clubbers and Turtle Tamers, MP stands for Muscularity Points. For Pastamancers and Saucerors, it's Mana Points. For Disco Bandits and Accordion Thieves, it's Mojo Points.

MP are consumed when your character uses skills. MP can be regained via certain items, or by resting at your campsite.

Inebriety
If you drink too much, you will be unable to adventure until the following day.