Halo 3 Im confused about skulls and Solo and Campain?

[furnaps08]

Ok Im currently playing through the game on solo Mode.

Do the skulls do anything but give you a 10 point achievement in solo?

What is the difference if I play Solo Or on campain?

Are they both seperate?

I play myself I dont have a gold subscription.

thanks
UPDATE::

Also Do the skulls effects work in the solo game or only in campaign? And how do I turn them on?

I can tdo campaign by myself rite? and its the same as solo?

its confusing to me.

 

[Fox Racer30]

Each skull has a different effect on the gameplay itself

As for the difference in co-op, or solo, well, for facevalue, the game is easier (as theres double the Human intelligence (we all hope =P) &, for example, if you die, & your teammate survives, it will allow you to respawn, without going back to your last checkpoint, but if you *BOTH* die, you revert back to the checkpoint, you can combine solo & co=op (i'm doing legendary with a friend, & im helping a friend halfway through herioc. I did normal, & easy solo)