Inner Fire
The barbarian inner fire is what fuels the power for barbarians to use their abilities. Berserks, dances, and War Stomp, magic resistance...pretty much the works. You lose inner fire by using your barb abilities such as dances,  berserks, war stomp and of course, by dying. Barbarians no longer take inner fire hits when mages cast spells on them (thank god).

Inner Fire Recovery
Inner fire loss is replenished automatically over time and by killing creatures. IF is gained on the killing blow, and only if it teaches your weapon. No matter what the creature is and no matter what the weapon is, as long as it teaches your weapon you will gain the same amount of inner fire with each kill. Even a 100th circle barbarian with a sling in goblins. Inner fire of course also comes back in time regardless of whether you are killing things. About every 2 minutes your IF recovery will pulse, giving you back a certain amount. As you become a more powerful barbarian the amount of IF you will recover with each two minute pulse increases. There is a cap on how much you can recover, and at least in The Fallen its 150th circle. This is probably the case in DR Prime as well.

Inner fire pools do not change. A 100th circle barbarian and a 10th circle barbarian have the same inner fire pool. What changes is how much inner fire you have to draw on to control your dances/berserks/etc as you get bigger. A more powerful barbarian with nice even stats can draw on their Inner Fire more effectively than a smaller barbarian with lopsided stats.

Checking Inner Fire and Inner Fire Levels
IF levels can be checked by kneeling (or sitting) and meditating on hand-held fire items or special barbarian fires (such as the fire fountain in the Mer'Kresh guildhall). Hand held fire items include lit cigars, tobacco, torches etc. The syntax is MEDITATE <item>. Chakrel items can also meditated on in the same way and I would still recommend investing in one. You can also meditate on a chakrel item standing up, but it incurs a roundtime. For more information on chakrel check out the Barbarian Items section. For more information on the MEDITATE verb, look at the Barbarian Abilities page.

When you check your inner fire the messaging will read "Surrounding your reflection is <inner fire level here>". For easier reading I'm just going to list the inner fire level. There are 13 levels of inner fire, and are listed from greatest to lowest.

1) a brilliant aura of pristine quality burning outward at twice your height.
2) a brilliant aura of beautiful flames extending nearly twice your height above your body.
3) a brilliant aura of beautiful flames extending a little more than one and half times your height above your body.
4) a brilliant aura of flames that extends over your height again above you.
5) a brilliant aura of flames that extends nearly your full height again above you.
6) a brilliant aura of beautiful flames extending a little over half your height again above you.
7) a brilliant aura of beautiful flames extending out around you about half again your height.
8) a bright aura of beautiful flames extending out around you just beyond the reach of your arms.
9) a bright aura of beautiful flames extending a quarter of your height above you.
10) a bright aura of beautiful flames.
11) a small aura of beautiful flames.
12) a tiny aura of beautiful flames.
13) a dim glow as if a flame lurked just out of sight.

Barbarian Magic Resistance
Every player of all guilds has magic resistance, but barbarians have by far the most. Especially when berserking or dancing. Magic resistance will attempt to do just as it says: resist magic. Spells directed at you or mages casting area effecting spells (such as zephyr) will have to challenge your magic resistance. The non-magical guild users (known as NMUs) have an extra benefit of magical resistance (a slight extra layer of magical resistance).  The negative side of this resistance is that beneficial spells cast on them will have a reduction of overall effect/duration on NMUs. NMUs are barbarians, thieves, and traders. The difference between barbarians and other guilds are that barbarians have dances and berserks which will directly give a boost to magical resistance (we call this, Barbarian Magical Resistance or BMR).

When someone casts at you and your inner fire overpowers the spell you will see:
Algion gestures at you.
Small motes of light flicker for a moment around Algion, then fade.

How is your BMR determined?
First off, a little on basic magic resistance. Your physical and mental attributes determine your base magic resistance. This is true of ALL characters, not just barbarians. Mental stats count twice as much as physical stats when determining base magic resistance. So if all your stats were equal, you'd get the same bonus from raising any 1 mental as you would from raising both agility and reflex or strength and stamina. This is why I have always urged barbarians to train in a method that does not seriously neglect any attribute. Mental stats are just as important!

Barbarian Magic Resistance is a bonus applied on top of your base magic resistance, giving barbarians extra mojo to resisting. Your inner fire level and any heightened boosts from dances/berserks are some key factors in determining this extra layer of BMR.

Inner Fire and BMR
Now I know after reading that you're thinking Phii's lost her damn mind. If all guilds have magic resistance, how come a non-dancing non-berserking barbarian has more magic resistance than a person of another guild with the exact same stats. Its because they don't have inner fire. Though all guilds have MR, only barbarians have Inner Fire. Inner fire acts as sort of a passive boost to magic resistance. Full inner fire protects at least 200% more than someone without inner fire. Having full inner fire protects significantly more than depleted inner fire. However, even that the very lowest inner fire provides at least a 50% boost to MR over a non-barbarian. Don't get too excited though, 50% is hardly noticeable to someone trying to throw a lightning bolt at you. Good thing we also have dances! On to the next boost.

Dances and BMR
Dances and berserks raise BMR. A lot. In fact, a dance such as Dragon Dance will more than double your ability to resist magic. Now remember what I said about your basic MR, and how its calculated in terms of your stats? Well the BMR boost added by dances goes like this: The higher the dance, the higher the boost. So whatever your basic MR is, you take a certain percent of that invisible number, and that is your added boost. The certain percent is determined by what dance you have active.

So lets pretend adding together all your stats (taking into account mentals count twice as much for MR), you come up with a basic MR of 10. You decide to Badger Dance. Now you've added the MR boost of the Badger Dance to your basic MR. Now as you can see below, Badger Dance adds a boost that is 50% of your base MR.

So 50% of 10 = 5.
5 is the boost you're going to get for dancing badger.
5+10 = a total magic resistance of 15.

Its all pretty basic math, but you can calculate it yourself with the list below.

Swan Dance - Little less than half. About 40%
Badger Dance - Half 50%
Cobra Dance - Little over half. About 60%
Eagle Dance - 75%
Bear Dance - About 90%
Wolverine Dance - 100%
Panther Dance -  About 110%
Dragon Dance - 125%

You won't ever actually get to see a number of what your BMR is (unless you suddenly get hired as a GM) but you can get a good grasp about how much your ability to resist is being determined.