Link building has its own province in the kingdom of search engine optimisation. Any SEO Company will agree that 90% of website ranking is due to link building and content. It can be done using different techniques. However it is important to do it in the right way or else it can lead to search engine penalization or blacklisting.

Google gave importance to links as it wanted to reward the sites which were actually good and were linked to naturally. However it hates manipulation and so it is important to have natural genuine links. Many SEO services providers stumble upon this aspect and pick up illegitimate methods of link building.
 
You will find many people selling link packages that are most of the time useless. They are created by linking to ads on classified directories or spam sites. Such links on your site can badly affect your ranking.

In earlier days reciprocal links were considered good and important. But gradually Google found many link farms manipulating the reciprocal links. Google developed algorithm to catch hold of such sites and now reciprocal links are not considered all that important.

It may many times happen that you link up to a site which is not indexed. This is of no use as Google values links only to web pages which are indexed. It is in fact a huge waste of time. You might ignore anchor text in the beginning but when you have 1500 links with no anchor text it is quite possible that your site can be taken up as spam by the search engine. It is essential to give relevant anchor text to all your links. Relevancy is another prime factor. Google is all about giving relevant results to the viewers. Links should be relevant to your web page if you want them to count for the ranking.

The key to have the best link building for your site is to do it carefully and organically. It is important for the links to be one way, relevant and from sites with good PR. Avoid link builders promising to do it for you quickly. In most of the cases such links are temporary and a huge waste of money.