A link can be an internal, external, affiliate, or ad. Basically, a link is a text that when clicked on takes you to another web page or website.
Internal links are links within your own website that takes you to another page within your website. A proper linking structure will help search engines crawl your website better, help the website visitor find what they are looking for easier, and promote SEO (search engine optimization).
External links are also known as referral links. External links are links from other websites that when clicked go to your website. External links hold value to help you rank your own website depending upon the value of the website where they came from.
Affiliate links are links that when clicked help generate revenue for the person that has the link on their website through the use of promoting their product or service.
Ad links are links within advertisements like the ones found in Google Ads that when clicked go to your website for a small fee.