Create Good Content, Link to Great Content
and Great Content Will Link to You.
Believe it or not having a good set of links coming to your site and leaving your site make a great impression on the search engines and your page rank!
Link Popularity
Link popularity refers to how many other websites link to yours and what content those sites contain. Link popularity goes on to include the link popularity of those other sites, etc. The search engines give your website a score based on the quality and quantity of inbound links.
Keep in mind, it does not matter the quantity of links coming in to your site if they aren't very good quality. It's better to have fewer high quality links to your site than many "junk" links.
Searching for Links
Looking for suitable links for your website can be a very time consuming project. It can also be very rewarding in increased traffic and page ranking. The best way to approach this is to look at competitors websites and see what kinds of sites they link to and what kinds of sites link to them. When you find a site that you would like to have link to yours it's best to place a link to their site on your site first. Then send the site owners an email informing them of the link you placed on your site, the relevance of your site with theirs and kindly request that they place a link on their site to yours. You can do this a little bit every day to build up a great link structure for your site.
Link Spamming
Watch out for this. Be very careful of finding websites that advertise the ability to place links to your website on thousands of other sites. Placing links just for the sake of placing links and getting a high volume of links coming in to your site is called Link Spamming. Search engines actually check the quality of incoming and outgoing links and websites found to be taking part in this "link farming" technique are penalized.
Summary
Building a great link program for your website can be tricky and definitely time consuming. It is something that you can do for yourself. But if you have the budget to hire someone to do it for you, that can take a burden of this process of your shoulders. Make sure to interview the company you hire to build your link program. Get someone who knows what they are talking about!
Be sure to read about Search Engine Friendly Coding and Search Engine Optimization and Marketing.