The HUD explained

Bars

The bars sit above your hotbar.

The hearts show your health.
The hunger bar, in drumsticks, goes down over time. When it is empty, you lose health.
The thirst bar, in blue drops, works the same way: if it is empty, you lose health too.
The blood bar, in red, takes the place of the XP bar.

Your health only comes back if you are not bleeding and if your blood, hunger and thirst bars are full enough. See Health.

Scoreboard

The Scoreboard, with the title DECIMATION, shows your Player Kills, Zombie Kills, Deaths, Clan, Humanity and Bottlecaps lines. If there is a bounty on your head, it also shows the amount.

The Scoreboard also shows alert lines:

“Bleeding” means that you are bleeding. A Bandage stops the bleeding.
“Zombie in…” means that you are infected. The countdown gives the time before you die.
“Broken Leg” is healed with a Splint or a Morphine Syringe.
“… seconds left” gives the time of immunity after you leave a SafeZone.
The quest line gives the current step.

Compass

A marked strip at the top of the screen gives your heading.

Center of the screen

The crosshair reacts when your bullet hits. A red cross means that your target is dead.
An arc around the center shows up when you get hit. It shows where the hit came from.
Above the hotbar, you see the name of your weapon and your ammo. While you reload, the word RELOAD shows in its place.
When your blood is very low, the screen turns red, your heart beats hard and the view shakes.

Player names

Under the name of each player, you see their clan, their humanity rank and their bounty if they have one. A player who wears a balaclava or a mask has no name above their head.