Quests are a search or pursuit made in order to find or obtain a reward. Quests are composed of objectives (find the cat, collect 10 berries), have a structure (defend, search), and serve the purpose of pushing the narrative forward or teaching players about the game.
Fallout 3 makes use of the classic quest structure, where-in you find various NPCs in the game world who when talked to generate a new entry into a journal/quest-log, from which the player can keep track of progress on the quest and overall, what quests they have taken upon. Shown above is an example of the quest log for Fallout 3, specifically showing the breakdown for “The Replicated Man”. As noted, we can see what elements make up the quest, and when completed the small box next to the progression entry will be filled in, as shown in the third item on the list on the right. The entries on this list tell the player how to progress on the quest and in good form, also present the options the player has to decide from on the quest. Quests are the basic building block of many games' goals, and as such are a pattern that any designer should take notice of when contemplating how to invoke goals.
When contemplating how to structure your quests refer to the RPG Design Patterns wiki as a spring board.
The lens is industry standard.
Navigation. (n.d.). patterns:questindex [RPG Design Patterns]. Retrieved May 9, 2014, from http://rpgpatterns.soe.ucsc.edu/doku.php?id=patterns:questindex