Player list window contains information on players in the game.
The window contains both players and observers, but can be configured to display only part of the at a time.
Today servers allow 32 players to exist in game - half playing, half observing. It is easy to see players from observers by looking at their player number (letter). Anyone below hexadecimal 'f' (15) will be player, others are observers.
The list contains useful information on other players, such as their name and login, statistics, hours spent playing, etc. You will get familiar with all details after some time of playing.