Artisan: They give the highest initial HP except for warrior, and provide the best A/D until level 235. They don't give any other particular advantages, and since the give quite a high experience penalty on other guilds due to high A/D, many characters do not go beyond level 30 in artisan. However, only a few characters do not go at least this far for the Hit Points that this provides.
Warrior: Give the highest HP until MxLV, and give the second best A/D. They critical hit far more often than anyone else, and their fighting ability allows them to deal out stacks of damage even without a critical hit. They are also second in line(equal with paladins) to be able to use items(in terms of level required. Doing large amounts of damage, and frequently critically hitting means that at least one of these guys is virtually in any party.
Paladin: A warrior of less ability, in areas of fighting, critically hitting, and extra swings, these warriors, are capable of minor healing to make up for their reduced prowess in combat. This skill makes them valuable in a party, as a fighter who can heal themselves does not require spell casters to keep him alive. Good aligned elves are often members of this guild, to give them some fighting ability. Not overly valuable, but can be useful depending on party makeup.
Ninja: The deadly ninja, able to both critical hit & backstab(though it has the worst backstab ability) is a valuable addition to any character. Often they chew up nearly all monsters in an encounter in a single round, and in a party, it is often necessary to frequently shift these characters to the back of a party so other members can get experience. However, their ability does come at a cost, getting penalties to other guilds up to high levels very quickly.
Villain: Quite a powerful guild, having access to minor healing spells & decent attack spells(element & morkal alchemy). They have a reasonable fighting ability and are reasonably good. However, given the penalty they give to other guilds, often it isn't worth having in your character.
Explorer: A fighting guild which is very good at moving around and have very high perception. These guys are virtually vital if you get lost. Being able to find their position quickly gives them a much better chance to survive if they happen go drop down a chute or accidentally teleport. They can also cast some of the less powerful resistances, though it is better to have a sorceror cast these spells for you. Quite useful.
Thief: Being undoubtably the best at opening chests, be they magically locked or armed with a nasty trap, every party should have at least one thief. Having magical entry at a minimum cost of 3 mana, and the only ones to be able to cast dispell magical lock(min 6) and destroy container(min 12) makes these guys the natural choice for disarming those horrid traps - especially those big corrosion traps that threaten to destroy your fighter's prize sword, or any other traps for that matter.
Barbarian: A cross-between the thieves & warrior's guild, results virtually a less effective thief - they have worse A/D, and are only one up on thieves with respect to their fighting ability. So, they are really just thieves, with a slightly improved fighting ability, but worse thieving & backstabbing ability. While they could be useful is some situations, often it is not be worth the extra time & effort joining this guild.
Mage: The sole guild to be able to charm monsters, this is another guild which no party should be without. When your characters get quested to return a monster, the mage can search and return with the beast - often avoiding a pinning situation. Having also an ability to cast some healing, movement and location spells also increases the abilities of characters in this guild. They can often provide an extra healer to a party.
Sorceror: The sole users of fire, cold, electrical and elements spells, members of this guild can also cast morkal damage, death & alchemy spells - this wide range of offensive spells provides them with the ability to kill creatures that other spell casters have to just attack with their weapons. Their ability to easily damage or destroy large numbers of creatures is unique to their guild, and this offensive ability is the often the best way to annhilate large groups of monsters. When encountering a large group of thieving monsters, a sorceror can cast a powerful spell to kill most of the monsters in one round - so the monsters are unable to steal items or gold and then run away. They also cast resistance spells for the lowest cost. Every party should have at least one, and seriously consider more.
Warlock: Being a general all round spell caster, warlocks do not focus specifically on any type of spells, but try to master as many as possible, which results in them often learning spells at a later level than other guilds. However, they provide characters with the ability to maximise their HP early on, and are just as effective as explorers at moving around the dungeon. They can cast all resistance spells, but drain more mana than sorcerors. They also have the best language ability - which is useful for finding out information off monsters. Providing characters with a wide range of spells, the most valuable of these probably being the movement spells, the warlock's guild is a good addition for most characters.
Cleric: The best at healing both themselves and others, clerics are virtually essential for a party's survival - especially when one starts to go deep into the dungeon. The ability to heal massive amounts of hit points with minimal amount of effort via restoration(min 7) and mass heal(min 8) is pretty much essential as the game progresses. The ability to resurrect(min 12) - which brings a character back to life, and to full health is reason enough to join this guild. At least one member of a party should be a cleric, because of the far superior healing ability.