Mana

Mana is an invisible energy that is used to create spells. As a general rule it is omnipresent on the material plane, although generally doesn't have any real effect on anything. The exception to the omnipresent is in an anti-magic field that renders spells useless. When a magic user uses up all his spells prepared for the day his eyes will dialate fully.

Wizards
Wizards prepare spells usually at the start of the day. They do this through a process they call the "Arcane Arts" which involves reaching out, metaphorically, and grabbing a clump of a mana and forming it into a spell which they carry around with them until they release the spell with a predetermined trigger, such as a hand gesture, a spoken word, or in rare cases even just a thought. In the etheral realm you can see the spells a wizard has prepared following him around. With a knowledge (arcana) roll of DC 20 one can identify what spells he has prepared, this roll has to be repeated for each spell to be indentified.

Sorcerers
Sorcers differ from Wizards in that instead of performing "Arcane Arts" they do what most scholars call "Innate Magic" even though there is almost little to no difference in the two. The primarily difference is instead of Sorcerers forming a spell that they can trigger like a wizard they simply will a spell into existence and it forms and triggers itself in that instance. There is much debate as to why Sorcerers are able to do this, people are certain that Sorcerers are born Sorcerers, they are not trained (though they can be honed). The most widely accepted theory is that Sorcerers are born with Mana in their blood and for this reason have a supernatural connectedness to the element. However nothing is set in stone, and many people still research the Sorcerers innate talents, although Sorcerers are generally pretty controversial with many people believing (sometimes rightfully so) that they are unstable and dangerous.

Psionics
Many would draw similarities to the power of Psionics and the power of Sorcerers, both energies claiming the source to be originating within the individual and wholly innate. But Psionics would strongly disagree that they use mana for their "spells". One reason being that all beings have the ability to be a Psionic, while not all can be Sorcerers. Psionics believe that every being has the innate energy of Psi within them to be a Psionic, and that they only need to unlock through meditation and self-contemplation.

Orcs
Orcs are naturally wary of magic users. It is a widely held belief in Orc culture that mana is made up of the souls of those that were not killed, and therefore were not absorbed by their killer. Most Orcs view the use of magic as disgracing the dead souls and angering their spirits, but there is a select few (usually magic users themselves) that believe this puts the leftover soul to a good use, and honors them by making their death practical rather than meaningless.