In sacred there are two main types of quests: Main Quests and Side Quests.
Main Quests are quests that you play in a specific order throughout a Campaign in Single Player (although you can play through the Underworld Campaign in a multi-player game). You must complete all of these quests to complete or "beat" the game.
Side Quests are optional quests that you can undertake by talking to NPCs throughout the game and can usually* be taken in any order at any time throughout the game in Single Player or Multi-player. There are multiple types of Side Quests:
Normal Side Quests: These are your average run of the mill side quests that you can take part in by talking to an NPC with a ?! symbol over their heads. They are usually very simple and quick to complete. You will earn experience, some gold and be given a reward by the quest giver upon completion. Usually the item given is a rune or a ring. Occasionally, a Set Item will also be given.
Implied Side Quests: An implied side quest is when you come across a situation that has the feel and look of a side quest but it doesn't appear in your quest log or have quest flags to go with it. Therefore they have no actual names but I have given them names in my quest list and some examples of these are Seraphim Rescue, Holiday Makers, Tarek the Minotaur, etc. Some people just consider these to be Easter Eggs; they have one thing in common with all side quests, however, in that the quest giver will give you a reward upon completion.
Series Quests*: These quests are unique in the fact that there are more than one quest linked together in a series. These are the only quests that must be done in order. You won't even find the starting NPC for the next quest in the series until you have completed the current step. Now these are tricky because sometimes you may encounter an NPC that gives you a quests and then immediately upon getting your reward they will once again have the ?! above their heads letting you know they have something new for you to do. But other times you just have to take a chance and speak to them again to find out. Worse still there are times that the next quest is given by a different NPC in a totally different area... therefore you have to go hunting for them and if you don't happen to run across them you would never know there was another part. (NOTE: Further down I will give a quick reference list of the Series Quests that don't have the same NPC or the NPC gives you another part in a different location. The obvious ones I have just listed normally in the quest lists.)
Character Specific Quests: These are some more average quests like any other except for the fact that they are only available to certain character classes.
Region Boss Quests: These are quests that only appear once you have reached 0 in the kill counter. The kill counter is a gauge that appears in the corner of your mini-map when you hit Tab, it shows the total number of enemies in a region and has a countdown of that number as you defeat enemies. When you reach 0 a quest flag will appear in that region to show you where the region boss enemies have spawned. They are a large group of unique boss monsters that you must kill to "Clear the Region". These monsters will always spawn at the same location in any given region.
One last note, side quest can also fall into more than one of the categories above. For instance, Intrigues is a series but also rewards a skill point at the end so it is both a Series quest and a Skill/Attribute quest.
Okay now that we have covered the different kinds of quests... here are links to lists of all the Main and Side quests in the game:
Special Side Quests
Here is a quick reference list for certain types of Side Quests (they are still listed in the Side Quest lists however).
Character Specific Quests
Good luck and happy questing!