Finding more and better magic items is critical to overcoming challenges and defeating enemies in Diablo III, particularly on higher difficulties. Magic find is a unique item property that can help you drastically improve your haul. It is tracked in the Inventory menu (Inventory > Details > Adventure).
But what does that really mean?
Magic find affects the quality of items you acquire from killing enemies (but not treasure chests, vases, weapon racks, or other environmental objects). It doesn’t give you a chance to get more items on a given kill – instead, it increases the chance that an item you find will be magical, and more potent than it would be otherwise.
When a monster drops an item, Diablo III randomly determines the item’s quality from a chart that includes item quality and the number of affixes present. The game randomly "rolls" on each property in the chart to determine which affixes your item will get. Your magic find score is applied as a bonus to these rolls.
For example, if a monster has a 4% chance to drop a 6-affix rare item and you have +50% magic find, it now has a 6% chance to drop that item.
Item quality is checked in the following order:
Using the above example, when your roll ‘misses’ a higher-level item quality, the item generator proceeds to the next lowest item quality in the chain (in this case, checking to see if you got a 6-affix rare, then checking for a 5-affix rare, and so on). Your magic find bonus applies to each roll. If the same monster has a 10% chance to drop a 5-affix rare item and you have 50% magic find, you now have a 15% chance to get a 5-affix rare item.
In a group, all players' magic find bonuses are averaged. So, if you’re in a four person group where two players each have +20% magic find and the other two players have none, all four players will effectively have +10% to magic find.