The World Wide Web is nothing without links; it’s how we browse-link to link to link and it’s how Google rates webpages. As a result we all want more links. A link is a kind of vote and we all want to be popular. On the web this means that we request, solicit, trade or even buy links to promote our pages.
Dan Thies explains the two types of links we need to understand. This is important stuff and needs careful consideration and rereading.