Loot is found almost everywhere. It can be obtained from quests, pickpocketing, stealing, dungeons, creatures and almost everything else. Most loot is in the form of septims, but may be weapons, apparel, or anything else valuable.
The best way to find loot is by clearing a difficult dungeon or camp. The best and most rewarding loot is generally found in the most dangerous or well-hidden places. If an area is occupied by several difficult enemies, it is more than likely a great place to find loot afterwards.
Loot is a very loose term, though. One common mistake is that loot is a reward from a quest, which is false. Loot is not a reward for completing a task, rather it is scavenged from dead bodies and chests. Many of the best equipment and ingredients are found via looting, mainly from bodies or pickpocketing.