Whenever you complete a dungeon (or trial if I'm not mistaken), if you queued solo you'll be eligible to commend a player for any reason you think it should have been commended (whether it was because he was a nice person and helped with the run, or because of his exceptional skill, or because he made a joke that lightened up your day, etc.) If you go to the last tab of your player window, you'll see the amount of commendations you've recieved (right above the beast tribes ranking). Upon getting a certain amount of commendations, your character is can receive achievements and/or rewards.