Links are necessary for website promotion but the links must be the valid one. High quality links could enhance the ranking or position of your website in search engines. Low quality inbound links are link that won't increase your search engine rankings. In the best case, low quality links won't have any effect at all on your rankings. In the worst case, low quality links can cause search engines to ban your website from their search results. When is a link a low quality link? Several points make a link a low quality link:

The link uses the nofollow attribute

Links that contain the nofollow attribute might help you to get direct traffic from the website with the link but they are totally useless for your search engine rankings. The nofollow attribute is a HTML attribute that has been introduced by Google, Yahoo and MSN. It allows webmasters to mark links that should not be followed by search engine spiders. The attribute can be added to link tags in the HTML code of web pages:

<a href="http://www.example.com/page.html" rel="nofollow">This is a link.</a>

If Google finds a link with the rel="nofollow" attribute, Google will not follow through to that page, they will not count the link in calculating Page Rank scores and they will not count the anchor text in determining what terms the page being linked to is relevant for.

The link is from an unrelated website

If the link to your website is on a web page that links to every X, Y and Z then it's likely that it won't have a positive effect on your search engine rankings. Try to get links from web pages that are related to your website.

The link is from an automated linking scheme

Some years ago, webmasters could blast their website URL to thousands of FFA (free for all) pages to quickly get hundreds of links. Search engines quickly discovered that and nowadays, links from FFA pages won't help your rankings at all. Shortly after that, webmasters developed automated linking schemes. Automated linking schemes usually promise hundreds of links in a very short time with very little work. As these linking schemes have been created for the sole purpose of cheating search engines, search engines don't like them at all. All major search engines have employees who actively seek for these linking schemes. Search engines know all linking schemes and links from these schemes don't have a positive effect on your search engine rankings. Worse than that, many search engines will ban your website if you link to a linking scheme website because they consider it spam.

The link doesn't go directly to your website

Some websites do not link directly to your website. They link to a page on their own website that redirects to your website, for example "http://www.example.com/redirect.php?3152".

You might get direct traffic through these links but they won't help your search engine rankings.

The link cannot be parsed by search engine spiders

Search engine spiders are very simple programs. If the link to your website cannot be found easily, chances are that the spiders will ignore the link to your site. Most search engine spiders have difficulty with links in JavaScript code. If the link to your website is hidden with a scripting language then search engine spiders won't parse it.

So it is necessary to get good rank or position in search engines you must have valid back links other wise it would be the wastage of time.