Link building is the process of creating links to your website. There are very many ways to do this and is usually time-consuming.
Self-Replicating Links is creating content or a tool that is so useful that other websites will want to link to your web page and when word of mouth follows others will do the same. An example of this is an MTG land calculator found on this page. This is usually a high-value link.
Directory Links hold a lower value where your website is listed in a directory of similar websites. The rule of thumb is that the more other links that are on that directory page the lower the value of that link.
Content Links are links that are within the content of another website that points to your website which holds a higher value.
Citation Links are links that cite research information from your website to include on their website.
Paid Links are frowned upon by search engines and you can be penalized for them sometimes because they are not ‘natural links’.