You may already know, but backlinks are one of the most important SEO tools you can use. Google and other search engines basically use backlinks as a way to tell which sites are deemed useful and relevant for certain keywords. In fact, the link profile for a site can be worth around 60% of the overall ranking results per month! So it is clear that it is important to have your backlinks up to scratch.
A good link profile will include links from a variety of relevant, high quality websites which link from their own pages to yours. This sends a vote of confidence for your website which search engines use to determine how good a site is.
It is possible to buy up hundreds of links to your site, but this is seen as a high risk strategy, which could get you into a lot of bother with Google! The quality and relevancy of your backlinks is more important than the number, so simply paying out to buy loads of links is not a good way to accumulate a tidy link profile. This is actually seen as a blackhat technique, and could easily land you a penalty for working against the moral code of SEO.