What does the "new campaign" button do in Starcraft 2?

I've just finished the campaign and I'm thinking about redoing the campaign to see other options and all. And I was wandering what the "new campaign" button does.

Does it erase my achievements ?

Does it reset the portraits I've unlocked?

There is some kind of score thingy... I made a thousand points. Does it reset?

Does the button do anything else ?

Starting a new campaign does not erase achievements, unlocked portraits, or decals.

All it does is start the missions over, from the beginning, so you can, for instance, take the alternate routes on the 3 "choice" missions (and see the after-mission movies, which you aren't privy to when simply replaying the mission via the archive).

It may also be the only way to lower your elapsed time for the "Hurry Up it's Raid Night" achievement. (On account of being able to skip some missions)

The new campaign button will just over write your previous campaign. All the achievements and portraits are saved along with your achievement points. The credits you got for technology and credits for beating each mission will disappear because its a new campaign.

You could always open a saved game from a previous campaign. But its just like overwriting a previous saved game, you are just overwriting your previous campaign :P