Here is how you add a category slug to your url in wordpress. Under settings->permalinks, type this under custom structure:

/%category%/%year%/%monthnum%/%postname%/

The more structure and clarity your url has, the easier it is for both search engines and humans to understand it. Search engines put a lot of weight on your url. Take for example these two addresses.

http://webplantmedia.com/?id=6
http://webplantmedia.com/business/2010/03/how-to-make-money/

One url is significantly clearer than the other. And search engines like this kind of clarity.

Adding on, it helps to add a category slug to your url. It’s the difference between having a folder dumped with a bunch of different files and categorizing all those files into relevant folders. One is intuitively easier to navigate through than the other. And again, search engines like this kind of clarity and structure.

Here is a list of other common structure tags

%year%
%monthnum%
%day%
%hour%
%minute%
%second%
%postname%
%post_id%
%category%
%author%