There is sort of a mini-game that is constantly running in the background. On the bottom right corner just above your right-click skill, there are always 3 symbols and a number. The symbol denotes what type of enemy, item, or event and number of times you need to satisfy the requirement. Once all 3 are highlighted from completing the requirements, you get a treasure drop equal to around whatever your character level is at. Here is a list of symbols and their meanings. For the enemy types, you can tell what an enemy type is by highlight the enemy with your crosshair.
Damage type used to kill enemies: Flame - fire type Lightning Bolt - electrical type Poison symbol - toxic damage Hammer - physical dam Circle looking icon with spikes - spectral type Paintball looking splat - any type of splash damage
Types of monster to be killed: Fang - beast Open hand - necros (ie. zombies) Pentagram stars - demons (duh! Sun with squiggly rays - spectral
Items pickup: Helmet - any type of armor (I believe this includes shields) Sword - sword type weapon Gun - gun type weapon Wrench and gear - any type of item modifications
Other icons: Illuminati looking pyramid with eye - turn in quest (thanks to wickedwild If there are other icons and you know what the req is, leave a reply and I'll update my post.
Oh one more thing. You can satisfy more than 1 req at once. For example, if you have a beast icon and a flame icon. If you kill the beast enemy with a flame type wep/spell, both req's will be satisfied.