This is your mailbox. This is where a multitude of things get sent to you. Some of these are friend or guild invites, rewards for tournaments or events, Honor scrolls you receive when friends use your hero are some of the mail you may receive.
Friends
This is where you can view the list of friends you have. Friends can be of great help especially if they are around the same level as you. You can summon their hero of choice to do one special attack per Battle. You cannot however summon them in PvP and some epic stages. Only on most of the Campaign maps.
Having a good hero and a lot of friends is also very helpful. If players like your hero and use it, you receive Honor scrolls for them using it. Although using a friend's hero can only be done so often like only one summon per Battle. Subsequent summons require 10 gems. And a specific friend's hero can be summoned once a day. You can refresh this by using gems. So go get some friends to use your hero in battle.
Leaders
This is where you find the players or guilds who top the charts on various criteria. These charts show the best of the best among the players and guilds in the game. You don't gain anything by topping the ranks but bragging rights.
Guilds
This is where you can join a guild or create your own for 500 gems. Being in a guild gives you the chance to befriend other players most likely even those stronger than you. You also get a guild chat tab that gets added to the chatbox so you can speak to your guildmates. The main focus of joining a powerful guild is to obtain rewards from the War of the Crowns tournament that is held weekly and starts every Sunday. This tournament can give hero tokens, free uses of the Heroic Portal summon, and some gold.
Tower of Pwnage - 28
The Tower of Pwnage is unlocked at level 28. It is a special dungeon that can be done anytime but has a very lengthy cooldown on getting another attempt. What makes this dungeon unique is that it has special rules in place that forces you to use every hero you have smartly.