The reason why backlinks are important in search engine placement optimization is obvious when you think about it. Any site that is truly an authority on its subject, or that many people enjoy visiting, will have thousands or millions of other sites linking to it across the internet. People will want to tell others about that site.

That's the kind of site that Google wants to display at the top of its search engine results, because it is likely to be what the people searching are looking for. Remember that Google search's main aim is to provide its users with the information that they were looking for when they ran a search. Google believes that this is more likely if it puts sites that a lot of people like at the top of the list.

When the search engines (led by Google) first began introducing backlinks to the algorithm that they use for figuring out what ranking to give to their sites, it was not long before savvy and unscrupulous website owners began trying to game the system for search engine placement optimization. They would build 'link farms' of thousands of sites, all linking to each other. In the beginning some of them were little more than a long list of links.

Of course, it didn't take Google long to see what was happening. They responded by not giving much weight to sites that were hosted on the same IP address. This means that there is not much point now in linking to your site from another site that you own, if it is hosted on the same server.

So then link exchanges also became popular. Webmasters would email each other saying 'If you link to my site, I will link to yours.' Services grew up providing search engine placement optimization networks where webmasters could exchange links automatically. However, after a while the search engines caught on to that too. These days, there is not much weight given to what are called 'two way links' - that is where two sites are linking back to each other in a closed loop.

Nor is there much benefit in paying for links. Google has actually talked about penalizing sites that are found to be doing this.

Use of the 'nofollow' tag on many blogs (especially those using WordPress) has led to a major debate about how much this matters to search engine placement optimization. Theoretically, it tells search engine robots not to follow the link. However, most SEO experts believe that most robots do at least follow these links in order to discover new sites, and some will also give weight to the site when it comes to search engine rankings.

Nevertheless, it may be better to spend time building links that will not have the nofollow tag. If you use the Firefox browser (which is recommended for any webmaster interested in SEO) you can download a simple add-on called NoDoFollow that will show you which links are nofollow by highlighting them when you view a web page. This means that you can easily see which blogs have nofollow links on their comments, for example.

To summarize, backlinks are probably the most important form of off page SEO that you can do for your site. However, you need to be careful about the type of backlinks that you build if you want to be sure that they will have the desired effect of search engine placement optimization.