Overview
League of Legends is releasing a new 5v5 game mode that's a cross between ARAM's emphasis on team fighting and the traditional Summoner's Rift experience with lanes and minions and, most importantly, the ability to pick your own champion.
You can expect the new mode to hit the client with Patch 25.10, and you can read more details about the new mode here.
The TLDR
5v5 mode with three small "lanes" in extremely close proximity and you pick your champions
10 minute average game time
250 HP for each team. First to 0 loses. 5 damage per champion take down. 1 damage per minion kill or minion push into enemy portal.
Neutral camps on both sides of the main lane that provide buffs
Can recall to heal and buy items
No Towers. No Nexus. Just fighting.