The 18 vs 18 battleground was not playable on the CBT. Due to his I can only provide you with confirmed basic information.
Once the battleground is released to the public the information will be updated.

You have to destroy the enemies guardian stone to win the battleground.
You base will spawn NPCs that are fighting for you.
You lanes are protected by KI controlled towers.
You can queue with a group of 18 players.
You have three lines.
Neutral Creeps are in the area between the lines.
Neutral Creeps give XP.
XP will increase your Warlevel.
Your Warlevel will increase your overall stats.
Your Warlevel is personal based – not team based.