That stands for "ID", so for instance, "#wrapper {foo}" means " ID="Wrapper" in the html.
There can only be one use of any particular "[B]id[/B]" in a web page. -Cannot be 'shared'. For values shared several/multiple times, you create a "[B]class[/B]", which in the Styles is "[B][SIZE=4].[/SIZE][/B]class_name {foo}" (note the period-dot before the classname). That way, if a several DIVs throughout the web page share the same declarations of color, line-height, font-weight, etc., they can draw this information from the shared "class".
Easy way to think of it, is "[I]ID[/I] is personal, there is only one" and "[I]class[/I] is like a classroom, there are many".
There is a reason why there are "ID"s and there are "CLASS"es instead of just one for all, -but I do not recall the reasoning. Just that it is so. ?
@stalepretzel59authorMay 22.2006 — #so an # selector is just like a class... but instead of applying to anything with that class, it applies only to the object with a certain ID?
[*] As a means to reference a particular element from a script.
[*] As the name of a declared OBJECT element.
[*] For general purpose processing by user agents (e.g. for identifying fields when extracting data from HTML pages into a database, translating HTML documents into other formats, etc.).