The HUD or 'Head-Up Display' is an interface in which important information about the ongoing game and the player's personal condition are relayed back to the player visually.
Health and Armor and Location
In the bottom left hand corner, the player's current health and armor are displayed. The bar below the health and armor tabs reveal the current player's location (if the map supports them.)
The bottom right hud block holds (working clockwise): Current weapon's 'clip' or 'magazine' ammunition, movement speedometer, primary grenade count, secondary grenade count, an icon for your current weapon, the ammotype for your current weapon, and the current weapon's ammunition reserves.
When playing the Engineer class, information about your constructed buildables will be relayed through the bottom right hand Engineer Menu. This information will contain health and ammuntion percentages for your Sentry Gun and Dispenser as well as displaying the current Sentry Gun build level.
Map Objective Information
The top center of the HUD offers information about the current ongoing game. This menu varies depending upon the gamemode being played, however the menu generally lists team scores, flag location, security status, the current map name, as well as a timer advertising time left before the map concludes.
The upper right hand portion of the screen displays 'kill messages'. When someone kills another player, that player's name is displayed in this area along with the cause of death (usually the weapon that caused the death) in icon form as well as the victim of the right side of the this icon.
Chat Stream Information
When a player uses the voice chat stream, a semi-transparent microphone icon will display his or her name on the right hand side of the screen. When a player uses the text chat stream, their messages as well as any gameplay changes (someone enters or exits the game, changes team, changes class, changes name, and so on) will be displayed.
Mousing over an enemy will display information about that player. This information includes the player's class, their name, and their health and armor percentages.