Basically, it's got:
Bottom left (top) five phase markers, showing which phase of the Game Round the player is currently in. This determines which units can have a turn at that time.
Bottom Left (bottom) are the portrait icons of the player's Officers who are in the current battle. Clicking on them selects and centers the screen on the officer. Right clicking on them does the same, but also automatically brings up the Give Orders display for that Officer.
Bottom Middle is the Menu. This determines what is shown in the information display window, or what types of actions you want the currently selected unit to be performing.
Bottom Middle (Right) is the information display window. This shows different things depending on meny selections, but is where you can read about the current status of units, change their ECM/ECCM settings, select scanner options, select weapons to fire, read about modifiers currently applied to units, see what rank, skills and abilities the pilot/soldier has, and all sorts of things like that.
Bottom Right (lower) is the mini map.
Bottom Right (upper) is the undecided. It will probably say what turn it is, or give some mini map viewing options, or something like that.