Trials are boss fights with their own story lines and no trash. Raids are collections of bosses tied together by a story line and the instances may include trash mobs that aren't a part of boss encounters.

Difficulty wise it goes like this: savage raids > extreme trials > normal raids (8 or 24 man) > normal trials. The gear rewards are mostly in line with the difficulty level. Although if the gear reward is on a weekly lockout or implemented in a catch up patch, the item level can be higher than the difficulty suggests. Omega Savage is opening today.