ID and CLASS

Hi all,

its possibly up to me that I don't understand the difference between ID and
CLASS. By now I think that ID is only for one occurence in a file and CLASS
for more occurences. Is this correct, or do I misunderstand the spec.

BTW, if I specify ID more than one time in a HTML file with linked CSS it
works as expected, every element gets the desired style, but my validator
tells me:
ID is already definded.

Thanks for any clarification
Yours
Volker
Internet Marketing Roose + Kroll
Berlin (Germany)
http://www.i-m-r-k.com
mailto: vkroll@i-m-r-k.com