From what I understand;

When you hunt a monster on an expedition there's a chance that it's "instance" will get added to the guild quest pool.

By registering this quest you make it available in the guild hall to take with others (which can make things easier). As you do the guild quest version repeatedly the level of the quest will rise, this will make the monster harder, but the rewards greater, meaning you'll want to do it more. I also think this will make certain carves more common.

What I meant earlier by instance was this. Let's say you get a rare expedition for a Velocidrome (amazing I know). In the information it may say the following.

Weapon: SAxe, CBlade, IGlaive Armor: Tri Series (Chest)

When the expedition becomes a guild quest you have a chance of getting excavated weapons and armor of the types depicted in the quest.

This can be especially good since if your friend is grinding for a good Insect Glaive you can now invite them on a quest to hopefully get a super awesome one.

As for Hunters For Hire that is thankfully easier to explain. In the Guild Hall (where you can talk to the hunters to hire them) there is a nice guildie holding a hammer in front of a gong. If you talk to her she'll give you options for what quests you want the hunters to do. These options include, rank, locale and target. Rank is quite simply what rank you want the hunt to be, low, high or G rank. Locale is what area you want the quest to take place in, this'll make more sense in a minute. Target has 2 options. Large Monsters, Small Monster/Item. By changing these options you can refine what quests you get to hire hunters for. For example by selecting Low, Ancestral Steppe and Large Monsters. I tell my game I want my hunters for hire to give me quests of any large monster that can be in Ancestral Steppe and go hunt them.

For information on what monsters can live where either check the monster hunter wiki or go to kiraneko (or check the MH4U Resource Thread to the right)

Hopefully this has helped.