Quests are your primary source of gold in Hearthstone. Every day a random one is chosen from a pool with higher chances of receiving one of the easier (and therefore less rewarding) quests. More difficult quests pay out higher gold amounts or even entire packs. Knowing how to maximize your profit by selectively choosing which ones to reroll is a very important part of growing your card collection.
There are a number of unique, unlisted quests that can only be completed once. These are detailed later in the guide. There's also an infinitely renewable quest that rewards you 10 gold for every three wins, up to a maximum of 100 gold per day. Neither of these take up space in your Quest Log and almost all can also be completed in Tavern Brawls, Ranked/Casual Play, or Arena.