what is broken link building ?

Broken Link Building is a different technique that SEO experts use quietly – it's also one of the helpful ones. Why not, when you're trying to fix a webmaster's broken link for him?

Broken link building may perhaps be the most effective, white-hat link building strategy in years. In particular, broken link building is appealing because the success of the campaign is directly proportional to how much good you do for the web. You profit only if you create good content to replace lost or abandoned content that webmasters still want to link to.

Broken link building is that it's catching link building opportunities that arise from some links on a site being broken.

Basicl you check  a website for broken links, e.g. using A1 Website Analyzer, and then inform the website owner of them telling him he can link instead to some aricles/pages of yours with similar/better content than the original links pointed to.