The in-game screen (or HUD) shows the following information:

Screenshot with HUD elements labelled

Element Description
Score Displays your current score and the total score needed to win.  In Smack Attack the score is Smack Points; in Gate Relay the score is laps; in Flag Rally the score is points.
Rank Your current position, from first to seventh.
Amount Ahead/Behind If you are in first, this is green text and an upward arrow indicating how many points/laps you are ahead of the second place competitor.Otherwise this is red text and a downward arrow indicating how many points/laps you are behind the first place competitor.
Current Weapon If you have picked up a weapon and it still has ammo left, it will display here.  Otherwise this area is blank.
Ammo A bar indicating how many shots or uses of the Current Weapon you have left.  If you have no Current Weapon, this area is blank.
Edge Radar Important objects in the world, such as competitors and Flags, which are not in view, will appear around the edge of the screen in the direction toward them.  For example, important objects that are behind you will appear along the bottom edge of the screen.  In the image above, B.T. Bruno is off the screen to the right.
Boost Meter Each time you perform a Flip (see Stunting) or pick up a Health Pickup, you will receive one additional Boost charge in your Boost Meter, up to five.  You can use Boost by pressing the square button for a short but powerful burst of speed.
Health Bar Indicates the amount of damage that your vehicle can sustain before “dying” and having to respawn.  Respawning will remove your Current Weapon, and all Boost charges.  In Smack Attack, the opponent that gets the “killing” blow receives bonus Smack Points.  In Gate Relay, you are respawned back at the last Gate that you successfully passed through.  In Flag Rally, all flags are removed.