Link development is one of the most controversial and often talked (written) about topics of the search engine optimization industry. Since backlinks are the most vital component of any search engine optimization campaign, and at the same time the most time consuming and consequently most expensive (assuming they are good quality links and not just random directory submissions and blog comment spam) part, inevitably, there are many service providers who offer inexpensive link building services in order to attract and impress potential clients.
Good high authoritative quality link development work focuses on quality rather than quantity. A well researched and relevant, good quality link is worth many times more than hundreds of free directory submissions or any links for that matter.
The fundamentals of link building are, have always been and always will be, based on good quality (i.e., useful, interesting, entertaining, educational) content. Because if there is no good content on your site that people can link to, it will be very difficult to convince them to do so.