Links are simple elements, but done incorrectly, they can slow up the movement of the viewer through your site(s), or even discourage them from continuing on ... which is NOT GOOD. They can also foul up the transmission of information, which will get you very few repeat visitors.
External Links
Links to other sites from within your site(s) are currency, and should bring you either links or banners on their sites in exchange. They should also be confined to a 'links' page unless necessary (for relevency of information). Always make sure that they are correct, working, and properly named. They should look something like this:
<A HREF="http://www.nameofsite.com/">name of site</A>
If there is a page within the site that you want to link to directly, it should look like this:
<A HREF="http://www.nameofsite.com/nameofpage.html">name of site</A>
I also recommend adding a target to the link, which will bring up the site linked to within a seperate browser. This will allow your viewers to go to the site linked to without actually leaving your site. However, bombarding your visitors with a new browser every time they click on an external link is not a good idea. Using the specific target of "external" will send all external links to a single external browser. A link with such a target would look like this:
<A HREF="http://www.nameofsite.com/" target="external">name of site</A>
Links to other Pages within a Site
These should never be absolute...that is to say, they should not begin with 'http://'. They should look like this:
<A HREF="/nameofdirectory/nameofpage.html">name of page</A>
or
<A HREF="/nameofpage.html">nameofpage</A>
or
<A HREF="./nameofpage.html">name of page</A>
If you are building them on your own server and need to have a more relevant path so
as to be able to use them/view them properly.
Internal Links
These should be named in a manner which makes sense, and should also not be absolute. They should look like this:
<A HREF="#nameoflink">name of link</A>
<A NAME="nameoflink">
When linking to an internal link on another page within your site, it should look like this:
<A HREF="/nameofpage.html#nameoflink">name of link</A>