Link building is the technique of gaining links to a website or webpage. As the number of links has such a powerful effect on search engine placement, link building is a key component of Search Engine Optimization.
Common approaches
Internal
Ensure that the most valuable pages within a website are linked to the most. There is no point in linking to every page equally highly, or any positive emphasis will be lost.
Ensure that if you control many websites they link to each other in an appropriate manner. For example, a different country or region site. This is usually good practice for end-users in any case, as they will often want to navigate between your different websites.
Social & viral
These approaches are among the most powerful but can be difficult to get right. They work best for material that people want to know about, and poorly for material where you typically need to interrupt users to pay attention:
Create highly-attractive material which people will find interesting by itself. Good examples are blog articles, free tools, infographics, and videos.
Link to your content using social platforms, such as Facebook and Twitter. This makes your content available to people who want to know about it, via a platform which encourages it to be circulated by itself.
Encourage blogs, forums and chatrooms to discuss and circulate your content.
Add the facility for users to circulate your pages via a control on your page (e.g. ShareThis).
Good websites with the public appeal will obtain links automatically. Accelerating this process is usually an exercise in encouragement.
Traditional
Submit the website to directories (free and paid).
Submitting desirable material to leading websites for your industry or sector. A link from these sites can be very valuable, and encourage more people to link to you.
Submit Press Releases or other articles to other websites, ensuring that they contain a link back to your site.
Engage in partnerships with other websites to link to yours.
Commercial providers
Many companies offer commercial link-building services. Many techniques for link building are frowned upon and some may even harm your website or professional reputation, so exercise caution.
In particular, anyone offering volume link buys (“5,000 links for only $100”) should be given a wide berth. The links will be, at best, worth nothing. Companies which will manually build links through reputable (‘white-hat’) means can be valuable.