Skulls are items that if found, unlock an altered version of gameplay in campaign.
In Halo 3, there are gold skulls and silver skulls.
Gold skulls unlock achievements, and earn bonus points in campaign scoring if turned on.
Silver skulls do not unlock achievements, and don't earn you any bonus points in campaign scoring.
In order to find these skulls, you must start the mission from the beginning on a difficulty of normal or higher.
That said, I will give out the location for all nine of the gold skulls.
I've found a video on youtube that can assist anyone skull hunting.
Click Here
One additional note: The "Catch" skull can only be obtained if the standard Wraith is not destroyed.
(It's okay to destroy the Fuel Rod Cannon Wraith however.)
In Halo 3, there are gold skulls and silver skulls.
Gold skulls unlock achievements, and earn bonus points in campaign scoring if turned on.
Silver skulls do not unlock achievements, and don't earn you any bonus points in campaign scoring.
In order to find these skulls, you must start the mission from the beginning on a difficulty of normal or higher.
That said, I will give out the location for all nine of the gold skulls.
I've found a video on youtube that can assist anyone skull hunting.
Click Here
One additional note: The "Catch" skull can only be obtained if the standard Wraith is not destroyed.
(It's okay to destroy the Fuel Rod Cannon Wraith however.)
Thanks that helped ALOT!!! God I feel bad because I never tried to find skull locations on the internet. 
