1 Armour, displays how much armour you currently have. When you are shot or damaged, some of the impact is absorbed by the armour you wear. Picking up armour or armour shards increases your armour.

2 Health, displays how much health you currently have. When you are shot or damaged, some your health will be reduced. Picking up health packs or using health stations increases your health.

3  Ammunition,  the amount of ammunition in your current weapon. Reload (default key R) to use some of your reserve ammunition. This indicator is either on the weapon or in the lower-right corner as indicated by the number 6.

4 Item Pickup, briefly changes to show the item you just picked up

5 Score and Flag Status , CTF and Team DM multiplayer only and shows your teams' score and the flag status in CTF

6 Timer, multiplayer only and shows the amount of match time remaining

7 Score, multiplayer only (not CTF) and shows your score and/or the team scores

8 Objective, when you receive a new objective, it will briefly appear here. To review your objectives during a game, press and hold the Objectives/Scores key (Tab by default).

9 Crosshair/Reticle/Cursor, your reticle generally assists in aiming. However, as you approach other personnel their name appears next to the reticle. When you approach a computer panel or other graphical interface, your weapon lowers and your reticle becomes a mouse pointer you can use to interact with the information on-screen.

10 Transmission, in singleplayer this alerts you to an incoming transmission,

11 Powerup, multiplayer only and shows the currently active power or powerups

12 Flag status, multiplayer shows if you are carrying the enemy flag

Some extracts from the Quake 4 Manual found on the Game DVD/CD