If anyone has any suggestions to add, feel free.
Martial:
Warrior: The guy you'd want at your side in mass combat, or if you want to work as a team with something. They're the melee team-player of sorts, while they may not be as actively skilled in combat with a sword as a fighter or swordsman, they truly shine when working as a unit with other people. If you want a guy who can take on three people at once with some skill, or take on one guy while working with three others, this is your guy. Warrior is more of a lifestyle and innate than Soldier, which is learned. You'll see that divide a lot.
Soldier: Pretty similar to a Warrior, but more learned and drilled. It's a class earned through training and implementing skills, as opposed to the Warriors inherently having the nature and freely refining. There's more discipline and technique involved than Warrior, and it is slightly more general in that a Soldier could be a guardsman, a mercenary, a recruit in some Lord's army, etc.
Fighter: Exactly what it says on the tin. They're good fighters, but not as good in mass combat as a warrior who's trained for such. They can use every weapon with a good amount of finesse, although up against a weapon specialist with their weapon they wouldn't do as well. They're strictly middle of the road, good at a number of things they do but there are people who do it better.
Weapon Specialist: Essentially a Fighter except they focus entirely in one type of weapon, whether that be sword, spear, etc.
Duelist: Opposite of a fighter - they excel in one on one combat but flounder when up against multiple opponents. Similar to a warrior, except when it comes to one on one, they get bonuses of similar size as the warrior when in mass combat.
Magical:
Sorcerer - Inherent/innate powers from any school.
Witch - Unisex term. Part alchemist, part enchanter, part Ritual Mage, etc. Tends to have a familiar and such, works well in threes, so on. There's supposedly "good" and "bad" powers, but it's really up to the witch in question. The famous Wizard Meeryn started as a Witch.
Mage - A generalist spellcaster. It's the Magical equivalent to the Fighter.
Blood Mage - Blood magic involves the sacrifice or use of blood for magical power. So basically, you can do a lot of things with blood. Any blood, the more magical in nature the better. Vampires make great Blood Mages, as do Blood Priests.
Ritual Mage - Ritual magic involves the use of rituals for magical means - spoken words of power, runes, use of material components, and the like.
Chaos Mage - Elder Magic.
Necromancer - Undeath.
Elementalist - Fire, Air, Metal, Darkness, etc.
Summoner - Summons things.
Mystical
Priest - Granted spells from the Gods - most of the time, these spells are simply more powerful versions of the spells of other schools, although Divine magic is cast virtually at will and refreshed only through devotion to the god in question.
Shaman - Similar to a Priest, although less dedicated to a single God and more of a belief system as a whole. Generally, Shamans are to Priests what Warriors are to Soldiers or Chaos Mages to Ritual Mages.
Seer - One who Sees. This can be the future, the past, within, without, etc. It does not rely on sight - it is more of the ability to perceive the whole from a single part.
Monk - The Mage/Fighter of the mystical class.
Projector - Projective abilities consist of evocation, creation, transportation, and necromantic abilities, using what would commonly be known as "chi" or what-have-you.
Reflector - Reflective abilities consist of transmutation, illusion, and abjuration.
Intuitive - Intuitive abilities consist of divination, conjuration, healing, and enchantment.
Mental
Kineticist - Kinetic psionics include telekinesis, and other forms of mental control - pyrokinesis, etc. Similar to Elementalists.
Telepath -Telepathic psionics include clairvoyance, telepathy, empathy, and the like.
Metabolic - Metabolic psionics include healing, biokinesis, vitakinesis, abjuration, and transmutation. Similar to Inituitive Mystic, but mind-based.
Creative: Creative psionics include creation, summoning, and evocation.
Mundane
Farmer, Smith, Accountant, Baker, Miller, Bartender, Cook, Lord, etc.
Mischievous
Thief - Ranges from pickpockets to vault-crackers to second-story folk.
Assassin - Not Assassin's Creed style.
Thug - Mugger, enforcer, guard, etc. Muscle.
Gambler - Professional gambler, card shark, bullshitter, people-reader.
Mixed
Knight - Exactly what it sounds like.
Bard - Musical mage, traveler, poet, student of life.
Ranger - Woodsmen, scouts, hunters, etc.
Paladin - Divine versions of the Martial class.
Sailor - Part Mundane, part Martial, part Mystic, sailors are a class all on their own.
Medical
Herbalist - They make healing things with herbs and other ingredients. Think poultices, soups, etc.
Bonesmith - Bone doctors.
Leech - Your average doctor.
Poisoner - It is to Herbalist what Ritual Mage is to Chaos, Soldier to Warrior, etc.
Surgeon - They cut holes in you and sew you back up.
Misc
Anything not covered that isn't a Prestige Class
D
D-level prestige classes are closer to "advanced levels" or "master levels" than they are to true "prestige classes".
Examples include:
Healer
Rouge
Specialized Mundane Class (Swordsmith, Apple Farmer, Leader, Speaker, etc)
Spellwright
Illusionist
C
Fanatic
Heretic
Archive
B
Adventurer
Alchemist
Nuke Mage
Free Mage
Champion
A
Racial Paragon
Enchanter
Prophet
Saint
Master
Wild Mage
S
Wizard
Bodyweaver
Defender of the Faith
Elder Mage
O
In a sense, it's where you go after Paragon 3, Wizard 7, Level 11's, so on. There's really no set criteria - maybe, over sixty years, Bob the Peasant 3/Merchant 1/Soldier 2/Apple Farmer 6/Brewer 6 accumulates enough knowledge about the art of apple cider that he gains a level of Legend. Or some kid, out of ignorance to the laws of Magic, invents a whole new Runic Language dealing with finding things because they lost a toy and did Something to find it.
G
God levels. Before you even ask, hahahahahaha NO.