The word “Links” means referring a website in web . The referral links can be of two types

1. Incoming Links / Back Links
2. Outgoing Links / referrals

Back links are most vital because they are supposed to be a measure of the popularity of your web site among the Web audience. There are good and bad back links. Good back links are from reputable places or renowned  websites – preferably from sites with alike contents and  theme. These links help to gain  search engine ranking. Bad back links come from non authorized places like the sites which differ in theme and contents  and the sites that do business with links .  You must avoid to get links from those sites. If there are a few  back link with your site , don’t upset  - buying links is an established practice and if you are serious about SEO, you must need to consider it. To start with you  should consider some free alternatives like  Web directories, for  http://dmoz.org  or  http://dir.yahoo.com.

After that you must find the relevant sites which are good in internet and maintain updated contents for its users . Then your job is to contact with the webmaster and asking the back links from their sites in return you can provide back links to that websites as well. It is a continuous job and monitor your back links in some regular intervals.