Backlinks?

[Release4]

They can also be called Inbound links, these are the Incoming links to a web site.
Search Engines use these links to determine the popularity of an Website, depending on the number of backlinks.
The Backlinks get their importance value first from the Topic, if the site that has a Backlink to your site, also contains content on the same topic as your page, engines will consider your page for good information site.
The higher the Rank of the Website containg a link to your site, the better the results for shared keywords, for both sides, if however you're links are irrelevant to the contents of a site, engines may lower your page rank.

You don't need 100+ links, 10-20 High Quality relevant Backlinks will boost your rank by atleast 40%, the rest of the ranking is being determined by other factor, like relevancy and quality of the contents of the page these links are pointing to.

http://en.wikipedia.org/wiki/Backlink

Thanks for the short explanation, loud and clear!

However, where can you find these inbound links? Do you always need to buy them or can you build them yourself?