Interface

Personal info: Info like your level, tactics, health, mana/orison
Buttons: Buttons to open windows like friends, inventory, party, combat skills, spells and mini-map.
Quick bar: Items placed here can be quickly used. Contents of potion box are shown on the right.
Combat: Change your combat state by pressing tab or clicking the button. On the right you can pick a combat skill.
Events/War: Pressing these buttons will show more info on events and war.
Mail/Game menu: Open in-game mail or game menu to change game options or log off.

From the help built into the game client.