HP
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 For information on hardcore permanents, see ascension.
HP
For those of you who have never played a role playing game before, HP stands for Hit Points. It's a measure of how much physical damage your character can take. Hit Points are directly based on how much Muscle you have, although Muscle classes inherently have 50% more HP than everyone else.
Your HP are displayed in the left pane in the form of a fraction: X / Y (underneath the HP heart). X shows your current HP, and Y shows your maximum HP. If the ratio of X / Y is less than 10%, the entire fraction will be red; otherwise, it will be black.
When your HP are reduced to zero, you cannot adventure until you regain some, either by using healing items or resting at your campsite. You also spend a few turns Beaten Up, which effectively reduces your primary statistics by half. You cannot go below zero HP, and you cannot die.
There are many weapons, pants, hats, and accessories to equip to increase your total HP. For a comprehensive list of everything in the game that affects your maximum HP, see HP Modifiers.
There are some effects which can reduce your Muscle and thereby your maximum HP, but your HP can never be less than what you would get with just your unbuffed Muscle. (Known to be incorrect.)
Resting in one's campground, using various items, and taking along specific familiars will also restore some of your HP. For a comprehensive list of everything in the game that restores HP, see HP Restorers.
Try to adventure with 0 HP yields the following message:
You can still, in fact, vacation though.
Contents
Notes
Calculation
 Your HP ceiling is affected by several effects and skills. The calculations, however, were changed on 25 Jan 2012, and many bonuses that used to be multiplicative are now additive. Furthermore, all percentage bonuses DO NOT multiply flat bonuses to HP, as they used to do.
 The calculation now works as follows:
HP = (Absolute HP boosts) + (Multiplier) * (Buffed muscle + 3)
 The multiplier is calculated by adding up whatever Percentage Increasers you may have.
 What this means in practice is that, when trying to maximize your HP and wanting to choose the best outfit for that goal, you may have to balance between equipping items that give more Muscle, or items that give more HP. Which gives you more HP depends on your HP percentage multiplier, and on your total Muscle  and, sometimes, on the base muscle as well, since percentage bonuses to Muscle depend on the base.
Example
For example.
 Chester's Muscle Shirt gives +20% Muscle.
 Sea Salt Scrubs gives a flat +400 HP.
 In order for Chester's shirt to give more HP than Sea Salt Scrubs, a 20% Muscle bonus would have to give 401 or better HP.
 So, with no +HP% bonuses at all (i.e. nonmuscle class with none of the three passive skills giving HP% bonuses), +20% muscle would have to add 401 buffedMuscle (and thus the player would need a BASE muscle of 2005).
 By contrast, a muscle class with all three +HP% skills, and thus the full bonus of 90% to MaxHP, would only need a base (black) muscle of (2005/1.9) = 1055 for Chester's shirt to give him more HP than Sea Salt Scrubs (because the 20% bonus translates to +211 to buffedmuscle, which then multiplies by 1.9 to translate to +401 HP.)
History
 Prior to January 25th, 2012  HP was calculated by the following formula:
Max HP = S + CEILING( A × CEILING( G × CEILING( R × MAXIMUM( FLOOR((M + 3) × C) + P, FLOOR(m × C) ) ) ) )
Where:
 M is your buffed Muscle
 m is your base Muscle
 C is 1.5 if you are a Muscle class (1 for other classes)
 S is your HP bonus from Slimy Sinews
 P is the total of other direct HP bonuses you have (e.g., from a bejeweled accordion strap)
 R is 1.25 if you have Spirit of Ravioli (1 if you don't)
 A is 1.1 if you have Abs of Tin (1 if you don't)
 G is 1.05 if you have Gnomish Hardigness (1 if you don't)
Assuming your Muscle is not suffering from a net negative effect, the formula can be approximated using:
Max HP ≈ GAR[(M + 3)C + P] + S = 3GARC + GAR(CM + P) + S