How do I link to part of a page? (hash?)

You use an anchor and a hash. For example:

Target of the Link:

 <a name="name_of_target">Content</a>

Link to the Target:

 <a href="#name_of_target">Link Text</a>

Or, if linking from a different page:

 <a href="http://path/to/page/#name_of_target">Link Text</a>

Just append a hash with an ID of an element to the URL. E.g.

<div id="about"></div>

and

http://mysite.com/#about

So the link would look like:

<a href="http://mysite.com/#about">About</a>

or just

<a href="#about">About</a>

If there is any tag with an id (e.g., <div id="foo">), then you can simply append #foo to the URL. Otherwise, you can't arbitrarily link to portions of a page.

Here's a complete example: <a href="http://example.com/page.html#foo">Jump to #foo on page.html</a>

Linking content on the same page example: <a href="#foo">Jump to #foo on same page</a>

It is called a URI fragment.