Quests are challenges with a storyline that offer rewards and incentives for completing tasks.
There are no cooldowns on Quests, so you could do as many or as few as you want in a day.
The majority of Quests can be completed 5 times.
Rewards differ depending on the Questing line and which Quest you are completing. However, most often the rewards are Quest Points and ingame currency.
At the time of writing, all Quest NPCs can be found at spawn. It may be difficult, but part of the challenge of the Quests are finding the right characters to complete your tasks. They do all exist, so look hard!