id is more specific...class is more general
Id will apply to the first element on the page that has the right id..
class will apply to as many elements on the page that have the right id
Also, ID's override classes..
i was just reading somewhere about it...i forget where tho :)
<div id="blue" class="red">blue text</div>
<div id="blue" class="red">red text</div>
and css
#blue {color: blue}
.red {color: red}
first line example above will be blue...second line example will be red because id only applys to the first element with the specified ID
<edit> Error is prolly right...my example might not be exactly true..its just how the validity works...</edit>
hope this helps!