There are very few items in Halo but getting any of them can make a big difference in battle. Just walk over any of them to instantly use them. They are usually out of the way and hard to get to. Memorize their location on every level.
The active camouflage is a clear pyrmaid with a light blue sphere inside it. It makes you almost invisible to the other players. Basically useless if your playing with gaydar on, otherwise its very powerful. Use your new found invisibilty to sneak up behind players and introduce your gun to the back of their skull. Beware that you are invisible but a player with a good eye can still see you coming. Also if you fire a gun while cloaked it will make you visible to the other players. Pick up the active camouflage as soon as you see it.
The health pack is is a small white octagon with a red cross on top of it. It will instantly restore your health bar to maximum. It will not restore your shields. Its not really a big deal, but when you get the health pack it will make a white flash across the screen. Although its very fast it might be enough to throw you off in a fight if you accidently walk over one.
The over shield is a clear cube with a red sphere inside it. It will triple the damage your shield can take. Very useful to have when your fighting any amount of enemies. When you pick up the over shield, you are invincible to all fire while it powers up. Remember that the over shield will automatically deplete at a slow rate in mutliplayer (this doesn't occur in single player missions).