SkullsThe Halo 3 Skulls are actual 'skulls' that can be found throughout Halo 3's campaign and when one is picked up it allows a new effect to be toggled on/off in the Campaign Lobby (this can be done by pressing X in the Campaign Lobby). When an effect is toggled on it will be applied to the next campaign game you play. These effects are designed to make the campaign more difficult when they are toggled on but there are also a few effects that are there just for laughs.
Skulls can also be found on the Mythic multiplayer maps which unlock an achievement when picked up but don't add any new effects for use in the Campaign.
The campaign Skulls can only be found on the difficulties Normal, Heroic and Legendary and you have to start from the beginning of the level to find them, whereas the multiplayer Skulls can only be found in Forge.
The Skulls also have an effect on Campaign Scoring by multiplying the number of points you get for each kill. I have listed each of their multiplying values on each Skull's detail page; Silver Skulls have no effect on Campaign Scoring.
It should also be noted that Silver Skulls do NOT unlock an achievement when picked up.