Google considers quality and quantity of backlinks. Quality links are links from relevant sites and site with good PR as it has been mentioned above. You can also build backlinks by creating original and inetersting content which someone will find useful and decide to link to it, that is you'll get a natural link. natural links are considered the most valuable and powerful by Google.
Install Google Toolbar for your browser and after installation you'll see a section on the Google toolbar called "PageRank". Hover your mouse and you'll see the pagerank of that website.
links to your site/blogs that have multifunction such as increase PR, search engine position depends quality and quantity
For buying links you can check Buy, Sell or Trade forum at DP, but Google disapprove of it and consider unethical. Your site might be penalized or even banned for it. Be careful with it!
The way that I approach Back Links is, any link to your site is a good link. Most people would say that is wrong but if your link is on a site, no matter the Page Rank, and that site gets some traffic, there is a good chance that some of that traffic will transfer to your site. A One Way Back Link is a link leading to your site from another site. A Recipricol Back Link is a link leading to your site from another site and a link leading back to the other site from your site. There is also triangle back links but that may be a little advanced. The best way to get backlinks is ask for them. The more emails to webmasters you send, the more yes's you will receive. Most of which will be recipricol back links. There is a great place in this forum to get back links called the "Link Exchange" The best way to get "One Way's" is through your site content. If your site has quality content then your visitors will automatically link to it. That creates one way links to your site which in turn builds more traffic. This is a long process so don't expect to much to fast.