Link building is a very important form of self-promotion on the Web. You contact webmasters of other, related websites and let them know your site exists. If the value that you have worked so hard to instill in your site is evident to them, they will assist their own customers by linking back to your site. That, my friend, is the essence of link building.
Think of link building as your chance to build your reputation on the Web. As your website is likely one of your business' most valuable assets, consider link building to be a primary business-building exercise. Just don't make the mistake of believing it will result in instant gratification. Successful link building efforts require a long-term commitment, not an overnight or turnkey solution. You need to continually invest in link building efforts with creativity and time. Good things come to those who wait (and work smartly!).