You can break down external links into two categories: Paid and Unpaid.
- Pay-Per-Click Advertisers in the United States
- Banner Advertisements or Image Advertisements
Unpaid links are ones which are "free", that is, they do not cost anything. Unpaid links can be added to your site through various free external link resources, or by convincing others to link to your site based on compelling content.
If your site contains information which is authoritative, especially useful to many people, or matches certain keywords well, then there is a high likelihood that your site will appear in Organic Search Results, also known as Search Engine Result Pages or SERPS.