In a stylesheet you define general layout settings like font sizes, alignments, margins etc. with CSS.
If you don't know what 'CSS' is, go and buy yourself a book about that topic.
In OpenWebCMS? you can create one or more stylesheet documents. In a stylesheet document you define
a speaking name
an internet key (which is important to access the stylesheet via web)
if it should be accessible for users with 'read public documents' access. Then in the body you define the CSS code.
After creating your stylesheet documents you have to tell the system when to include them. You do that with a template (see (?Customizing Templates?) for more info). In a template you can include one or more stylesheets, that means you are allowed to use cascading stylesheets.
For example you can create one stylesheet document with all common definitions, and a second one which only includes special definitions for special types of content.
Then, in one template you can include only the common stylesheet, and in another template you can include the common and the special stylesheet.