For example the html
<a href="www.yahoo.com">free e-mail and news</a>
Is treated relative but
<a href="http://www.yahoo.com">free e-mail and news</a>
links to yahoo.com. Is that all there is to it, the only way a link is absolute is if it starts with http or https?
Yes. Please Refer to the below link, which explains the Hyperlinks syntax.
HTML BASICS: HYPERLINK SYNTAX – ABSOLUTE, RELATIVE AND ROOT-RELATIVE