[Solved] what does # signify

Hi

Excuse my ignorance. I'm very new to CSS (but i'm reading up). I understand that classes are declared in css with a point (e.g.)

#header .logo{
}

but what does a # signify? I've tried searching the forum but I obviously can't search by "#" and I dont know what's it's called. Pls excuse my ignorance ... any advice would be great.

Thanks
Giles