@G00785 farming is the act of getting gold, items, pets, crystals, myth stones. a build that puts its focus on farming is not very good in the Arena (PVP), and takes a lot of work to climb the floors in the Dungeons.
at lower levels, gold find and luck are important affixes for farming. putting Hero Points into the Fortune skill will help with farming too. there are a few Legend items that have a focus on helping a player farm for loot.
when you have played for awhile, you will find items with Nadroji, Mythical, Crystaline, and Eternalized Set Affixes. they help you with getting better loot and certain items.
the Luck and Item drop Shrines increase your chances of better loot on the maps where you find them.
the Fortunate, Treasured, and Dealer Perks help a lot with farming.
challenge maps are maps that have better loot than regular maps, but getting the loot can be harder, as there are challenge map affixes that give the monsters boosts, or make you weaker. Legend/Eternal Maps have their own map affixes, and they all are about boosting the loot you find, but they still have tougher monsters than regular maps.
look in the Codex for items that have luck, gold find, item drop, Fortune. Satyrs Spirit, Adventurer, Steigers Fortune are items that will help with farming. Stiegers Fortune is a Wizard item, and I think the other classes have similar items to help with farming.