Today's PBE patch includes a new Champion Mastery system, a new reward system where you rank up with each individual champion while playing and earn various rewards.

Here's more information from Riot's release announcement:

"CHAMPION MASTERY  We want to recognize you for playing the champs you love and give you feedback on how well you play them.

"With Champion Mastery, you now get points, ranks, and rewards for the champs you play. Now, if you want to show other players that you’re a dedicated Swain main, or a big cheese with Twitch, you can!  HOW THE SYSTEM WORKS

Finish a game of League and you’ll earn points for your champion based on your team’s performance as well as your own contribution. Earn enough points and you’ll level up that champ’s mastery level, giving you rewards as you progress.

Grades  Hover over your Champion Mastery points bar in the post-game screen and you’ll see the grade that our new system gives you. Grades are awarded based on your overall performance in the role along with the champion you’re playing and, crucially, where you’re playing them. Leveling Up  Check out your in-client profile, and you’ll see a new stat telling you your mastery score, a sum of all your mastery ranks combined. 

REWARDS We’re kicking off Champion Mastery with the first five ranks for each champion unlocked. Here are the crests your champion will earn as they rank up:
ROLLOUT

We’re kicking off the rollout with a PBE stint from 3/11. If all goes well, a couple of weeks later we’ll then launch a beta in a single region, focusing on Summoner’s Rift normal queues to make sure the feature’s stable for expanded use. We’ll read through your feedback and keep you updated with our progress, too, so keep an eye out for future posts after the service goes live!  Eligibility  All summoners who’ve hit level five or higher will be eligible for Mastery points. However, at launch, it’ll only support normal games on Summoner’s Rift.  The Future  This is just the beginning for Champion Mastery. As the system matures, we’ll unlock extra tiers and rewards as we build on the program! "

AIR Client animations -


IN-Game strings:

 "Make your champion display your mastery badge.  This will cancel any attack or movement commands."   "If you have unlocked a mastery emote for the champion you are currently playing, the command will play your mastery emote."