ARAM - All Random All Mid.
ARAM is played on the 5v5 map Howling Abyss, that map only has 1 lane, mid, and you can't use recall( recall is a mechanic in league of legends that every champion can use. You press recall and your champion teleports back to base after standing still for 8 sec) and you don't regain mana or health and after leaving base you can't use the shop. The objective is the same as on Summoner's Rift, destory your enemies nexus!
While in other gamemodes you can only pick champions that you own or the champions that are free to play that week, in this game mode you cna get any champion randomly. If you don't like the champion you got you can reroll. A player can have up to 2 rerolls and if your teammate rerolls a champion and you own it(or it's in the free to play rotation) then you can select that champion.
Skin Boosts
This mechanic is only available in ARAM. 1 person can pay 95 RP (Riot Points, LoL currency that can be bought with money) and everyone on his team will recieve a 1 random skin for that game. It's really cheap and fun way 5 people can try skins for 1 game.
My Honest Opinion
This is honestly really fun gamemode, but also it can make you rage. When you roll a great champion and dominate your opponent then it's really fun, but if that happens to you, you will be pissed. It's really good that Riot implemented this game mode, because it was a community gamemode before. This game mode was released It was released on 2012-06-29 and since then it has been updated and changed to become this cool and chill gamemode for everyone to enjoy.