what exactly does the destruction skill do?

To quote the Unofficial Elder Scrolls Pages, "The destruction skill is the mastery of the spell effects of the College of Destruction. Their spells harm living and unliving things, and include elemental damage, draining, damaging, vulnerability, and disintegration magical effects."

so does is increase the damage caused every time u increase the skill?

No, because how much damage a spell does is based entirely on the spell's construction - a spell that does 30 points of fire damage is always going to do more damage than one that does 10 points of fire damage - but how high your destruction skill is factors heavily into your actual ability to cast a given destruction spell.

so if it was higher then i would have more of a chance casting a destruction spell?

It depends on the spell, but, essentially, yeah. The higher the magicka cost, the higher your destruction skill needs to be. There's an indictation in the magic menu of your chances of casting each individual spell; the higher your destruction level, the better the chances that you can cast it.

so if i had a spell, lets say summon deadrath(i think thats how you spell it) and i only had 60 destruction, how much higher would it have to be to cast it?

Summon spells are conjuration, not destruction.

It would also depend on the time. The longer you want to summon something the higher skill you would need.

Any time you want to do something bigger, or longer, for a spell, your skill will need to be higher to cast it.

This is the equation that determines your chance to cast a given spell:

([Luck / 8] + [Willpower / 4] + [Spell Skill * 2.5] - [Magic Cost * 1.25] ) * ( 0.6 + [0.4 * Current Fatigue / Max Fatigue])

ah right, thanks again

Well what destruction does is it allows you to cast poisin spells damage health spells and that type of thing.