You pay for people, link farmers, to link back to your webpage with specific anchor text. The link farmers typically have a network of websites they can distribute this over, offering you a few hundred or thousand links back to your site with your targeted keywords. Sometimes, the link farms even have good pagerank, which makes it even better.

That's the whole point of the Google backlinks - get as many links back to your site as you can, and you'll get good SEO.

And if you do this, and Google detects it, they nail you.