In search engine optimization the link building has its own importance because it brings the web traffic in the website. According to the experts of SEO Link building is perhaps the most time taking and a tricky task among all work. In this particular work there is so much of concentration; build up, secrecy needed and these revolves around link building strategies. Many sites that deliberately pay no attention to search engines on-page rules, and having links from sites that are non linkable and are not counted as relevant to the website. 

Talking about the Good links on the website we should mention that those links can be considered good links which come from social networking sites and user communities and forums. The SEO expert targets by purposely presenting the website or the content to these websites physically to get relevant links. If a website became popular getting a good page rank and a good range of inbound back link then your website reputation increases and as far as other websites are concerned they gives a link to your website or page content a “vote” and therefore it can be considered as a good link.
Talking about bad links then the search engines actually mean about some specific links as bad links if they joined or associated with a penalized site or any website which is getting a link back from such a website which have followed some unethical SEO strategies. These links are certainly not worth of linking and considered as bad links according to the search engines.