Mages
Mages are masters of  arcane knowledge.   Many years of patient study will be needed to allow the mage to control  very powerful and violent magics. An apprentice mage must be willing to sacrifice power in the beginning in the hopes of learning the more powerful spells.

A mage will have the power to summon monsters to their aid, move from place to place in the blink of an eye with the portal spell, harden their skin to protect from harm with stone skin, and much, much more.  

Single class mages get a more powerful arcane powers spell, higher ability to learn a spell, and the ability to cast a spell above their own level.

Clerics
Clerics are an important part of any traveling group. They are able to heal, bless, turn the undead, cast violent aggressive spells that are not matched by another PC type, and cast many other spells that enhance a parties ability to stay alive.  As with mage, a young cleric is weak and will progress slower then some of the other classes, but in the end the cleric is indeed one of the strongest and most able PC types. 

Single class clerics get a more powerful heal and restoration spell, higher ability to learn a spell, and the ability to cast a spell above their own level.

Thieves
A thief is a PC who does things in a round-about way. He doesn't always kill the mob to get the goods, he steals the goods from under their eyes. He doesn't kill the mob with a wild frontal attack with blazing swords, he attacks from the rear with a vicious back stab.  He won't plod through an area daring mobs to attack, he sneaks through and takes a peek so he knows what he is getting into. In addition a thief can detect and disable traps, call mobs out of a crowded room to allow for a quick kill, and pick locks. all very valuable skills for a party.. just watch your gold.

Single class thieves backstab better than multiclass  thieves.  As well, they can learn their skills better than multiclass warriors (less failures).

Warriors
You like hackin' and slashin' and rescuin' fair damsels, self health and welfare be damned? Then this is the class for you. You will learn fighting skills like bash and kick, and you'll have the largest selection of armor and weapons, even at low levels. Some of the powers that are not available to you are also available in potion (or scroll, once you learn to read magic) form that can be found in shops and on mobs. A warrior is powerful in his own right and will always be around when there is blood to be spilled.

Single class warriors get more hit points than multiclasses ones, and get more attacks per round.  As well, they can learn their skills better than multiclass warriors (less failures).

Monks
Monks are masters of harmony.  They are very aware of their internal balance and are resistant to things that upset that balance.  This characteristic does not necessarily force them to be neutral.  Over time their training enables them hit attack others with their bare hands and to kick things with amazing force.  Their focus on their being neutralizes any affect dexterity would normally have on their armor class. Very little is reflex anymore. As they become more powerful they become resistant and eventually immune to things that would disturb their internal balance.

Monks are designed to be weak & equipment starved at low levels. At high levels their powers allow them to rival and even surpass the most powerful warriors.

Druids
Druids are priests that wield and worship powers of nature. They strive to maintain the balance of all living things and repair it when necessary. This system of beliefs requires them to remain neutral at all times. A Druid that is not neutral is lost, irrevocably. To compensate for this, they are given powers that no other character class can attain. They are resistant to charm at level 14, immune to poison at level 32, and immune to charm at level 60. Their spells include healing, harming, elemental offense and natural defense.  They can also call upon many allies from nature.

Elementalist
Elementalists are masters of the 5 elements.  They can specialise and be master in a few elements of their choice.  Elementalists tend to lack "utility" spells not related to an element, however, their attack spells can easily surpass a mage's.  Elementalists get better mana regeneration than all of the other classes.