Commendations is the term used to describe any meta-award you get in game. These apply to things like Ribbons and Medals that you get in game but not the achievements that you get while playing.
Ribbons are awarded for actions that happen over the course of a single game and may cumulative over that match. For instance, you can get a ribbon simply for getting a single headshot during a match or you can get a ribbon for getting 10 kills over the course of the match without dying. You will NOT get a ribbon for getting 20 kills over the course of two matches without dying.
Medals are awarded for long term feats and can, in some cases, involve getting ribbons. For instance, you could acquire a medal for getting 100 kills with a Lancer over the course of your career or you could get a medal for having obtaining the Killing Spree medal 10 times over your career.
