Search engine optimization, or SEO, is a combination of techniques which aim to improve the ranking of a website in search engines (such as Google, Yahoo, etc.). SEO is very complex but it can generally be divided into two parts: on-page optimization and off-page optimization.
On-page SEO includes factors like URL names, HTML tags, keyword usage, image optimization, and link optimization. While each of these on-page SEO factors requires a different task, the aim of them all is to make the website more “readable” to search engines. A search engine cannot physically look at your website to determine what it is about but the on-page SEO (such as having a high density of a certain keyword) helps the search engines read your site.
When we talk about off-page SEO, it is usually referring to incoming links. Search engines count each link going to your site (backlink) as a vote of popularity. The more votes of popularity you have, the higher your site will rank. However, it isn’t enough just to have a lot of backlinks- they also must come from relevant, high-ranking sites in order to be valuable.