Display
Toolbar
This bar allows you to control game features such as economics, as well as ending the game turn by telling it to generate 1 turn.  "Civ Econ Orders" is the way you control the economy of your whole civ as a unit.  Select any square that has a functioning economy (the village tiles) and then hit the button (see Economy).   "All Views" shows a list of all windows avalable, and is the best way to get back a screen that you have minimized.  Move is the button to push when you have selected a TF and want to move it.  Place the cursor where you want the unit to move, and then left-click the mouse. 
 
Map
This is the main window where much of the play happens.  The selected tile is outlined in white.  If a tile is selected the military units in the tile are shown in the Units Box (see below).  Only one representative military unit from each side is shown for graphical clarity.  The "village tiles" are the only economically active tiles in D5. 

Power Circles are the circles that surround the units in the map window; the color defines which civilization they belong to, while the Area of the circle indicates the strength.  They are also shown in the "Unit Box".  There is a minimum and maximum size for a power circle so the graphics look attractive, so for low-powered TFs the power circle may not be a good relative gauge of military power.
 

Units Box -- Task Forces/Units in Square
The Units Box (upper right in the GUI -- Shown at lower right below) indicates what units are in the square that is currently selected, and how they are organized into task forces.  Task Forces are a collection of units that have been grouped together to represent an army of units that move and fight together. The Task Force takes the movement of the slowest unit, but groups together the strengths of all of the units.  A large number of Task Force rearrangement and movement options are available when you right-click on any unit.

TF and individual unit military power  Power in the TF panel is indicated by 'power bars' that use the same principles to indicate power as the power circles on the main map.  The line length is scaled the same as the diameter of a power circle.  A bar that is 2x as long as another means the unit or TF has 4x the power.  Both individual units and TFs have power bars.  Power bars for units are under the unit graphic.  The TF Power Bar is below that for the units and centered in the bottom of the TF box.
 

Detail Frame
Shows detailed information on the selected square, such as population.  Will show more as the game evolves.
 
Information
The window displays the readout of the processed turn (details of battles, etc.).