The Call To Duty Quest
The Call to Duty quest is a special instance dungeon. The basics of a dungeon/instance is that it is a place where you and/or a squad can go to and kill certain elite monsters; at the end of the dungeon/instance you can kill the boss (essentially a super monster). It appears (from the inside) like a cave.
The Call To Duty quest is a quest that you get every x9 levels (with the exception of level 9 and level 49) for the call to duty quest that you'd usually get at level 49 you actually get at level 51, and there simply is no level 9 call to duty.
What's an FB?
An FB stands for Fu Ben. A Fu Ben (FB) is another name for a Call to Duty quest. Fu Ben actually means Chinese for "duplicate object" or "copy of", better translated here as "instance". For example an FB19 is the Call to Duty quest you receive at level 19.
What's the point of an FB?
There are many good reasons for doing an FB (Call to Duty). The first being the reward for the quests. You get EXP and SPIRIT from completing the quest as well as the potential for a drop in an FB. Drops usually mean something that appears on the ground when you kill something (typically coins or low value item), however in an FB you have the chance to get a mold (that can be used to manufacture legendary equipment) or three-star (usually called 3* or ***) equipment which is semi-rare and pretty good.
Tabs, Quests, and Wine
All FBs have tab(s) and if it is your FB it is important you get these, they can be gotten from various NPCs depending on which FB you are doing. These tabs are used when the tabber (person who has the tabs) talks to a pillar in the dungeon/instance. These pillar(s) can be found at the beginning of the dungeon/instance or at the boss the pillar is related to. The people who are assisting you do the FB require you to have the tab(s) in order for them to get EXP and SPIRIT. These tabs in-game are called Call to Duty: X. The X is for whatever type of thing your killing. For FB19 it is Call to Duty: Xenobeast and for FB51 its called Call to Duty: Wraithbeast.
For most FBs there are quests where you have to kill mobs (monsters) in the FB, in this case you will have to kill those mobs in order to complete those quests.
There is also Wine, an FB in which wine is used it is called a "wined FB". Wines can be manufactured or purchased from other players. Basically, wine clears the mobs so you can go straight to the boss. There is an NPC in the instance that you can give wine to that clears the mobs. For more information about wines you can see Aryannamage's guide to FB runs which talks about wines(I compiled this guide into a wiki): FB Guide
Squads
A squad is an important part of an FB. Having a good squad is crucial. Firstly you'll want to know that the lower level your squad is the greater chance there is that you get a good drop. You should probably have at least 1 barbarian (for tanking) at least 1 cleric (preferably 2) to keep your tank healed. You'll want a good venomancer who knows how to pull (also called lure - this technique involves attacking the enemy with your pet, stowing your pet, then resummoning it). From there you can add in some damage dealers such as archers, blademasters, and wizards; it might also be helpful to add in another cleric.
Maps and Navigation Through FBs
Finding your way through an FB can be difficult. I'd recommend that you'd go with someone who knows how to go through the FB. Some FBs are relatively straight forward however some can be complicated and require you to kill or not kill certain mobs to open a door.
Good luck
