The Fields area is where the actual game-play takes place. If you do not have the Show Fields on Game Start option checked under Misc. Options, then simply press the button to the lower right of the program, marked Show Fields as seen here:
This will bring up the game window that looks like the following:
On the left hand side, you will see a large rectangular field. This is YOUR playing field. On the right hand side, you will see 5 smaller rectangular fields. These are your opponents fields, that you can watch and adjust your own game accordingly.
In between your field and your opponent fields, near the top, is the preview images of your next two pieces, followed by an area to display the piece you are currently holding. Note that in ordinary TetriNET mode, you can only see the first preview piece, and piece holding is disabled.
Below those areas are text lines, containing information on the lines you have cleared, your current level, the averaged levels of all players, and the speed you are playing at, measured in pieces per minute.
Underneath those text lines is a box that displays all of the events that occur during a game, such as specials used on one another, lines added to one another, etc...
Partially underneath the events box and underneath your own field, is your specials inventory. If playing a specials game, this is where you will see the specials you have collected by clearing lines with specials on them. To use a special on someone, press the number associated with the player you want to use it on, seen in the caption of the field beside their name. Note that the keys are definable under Misc. Settings.
Below this is the game chat box. To bring up an area to send a game text message for the other players to see, press the 'T' key and type in your text followed by the enter key in the edit space that shows up. To get rid of the edit area without sending text through to the game, simply press the escape key.
If you wish to get a closer look at an opponent's field, simply double left-click with the mouse on the field you are interested in zooming in on, which will enlarge the field over top of the other fields, as seen here:
To return the field to the original size, simply double left-click on the enlarged field once again.