The hash mark signifies a unique ID, and if not already familiar with the various uses, you soon will be.
Written in a CSS statement, the hash mark identifies a style, or block of styles, that will be applied to a particular element and no other. Please note however, rules of inheritance may be a factor for child elements.
<div id="announcement-box">Important Information:</div>
In the above example, the CSS statement creates a set of styles for the unique ID "content" which, can only exist once per page, though may occur again as a "page-only" element, elsewhere in the site.
The simple rule to follow is: Use .classes if a style or set of styles, will likely be applied multiple times on a page. Use ID (#id) if the element is a unique component of the page and will not occur elsewhere on the page.
The ID is extremely useful as once applied to an element on page, that element can be manipulated using scripting, styled with CSS and/or targeted by hyperlinks.
Spiders LOVE standard hyperlinks!
On a page, the hash mark, or fragment identifier, allows targeting of content specific anchors. The following examples demonstrate possible usage:
A HEADER as a anchor: <h3><a id="grants">Grants</a></h3>
A HYPERLINK targeting the fragment identifier from another location on the same page: <a href="#grants">Grants</a>
A HYPERLINK to the same target but from anther page on the same site: <a href="page.htm#grants">Grants</a>
A HYPERLINK to the same target but from another website: <a href="http://www.example.com/index.htm#grants">Grants</a>
NOTE: Html 4.0 uses name to create anchors: <h3><a name="deprecated-by-xhtml1.1">Use ID for Targeting</a></h3>
Now using the very first example: <a href="http://www.www.example.com/page.htm#announcement-box">Loan</a> a hyperlink is created that targets the announcement box on the page.htm page which is styled to float left, be 16em in width, have a light gray background and render medium gray text! Yep, the old #hash mark is pretty useful! It can serve multiple uses all at the same time.
[edited by: Nick_W at 5:59 am (utc) on May 21, 2003]
[edit reason] no specifics / urls please [/edit]