In one line
A step that sends the browser from one address to the next: an HTTP status code, a meta refresh tag or a script.
An affiliate link rarely goes straight to the shop. Each hop on the way is a redirect, and there are three kinds.
- HTTP redirects: the server answers with a status code starting with 3 and a Location header naming the next address. 301 and 308 are permanent, 302 and 307 temporary, and 303 points to a different resource. Browsers can cache 301 and 308, so a second click may skip that hop.
- Meta refresh: a tag in the page telling the browser to load another address after a number of seconds, often zero.
- Script redirects: the page's JavaScript sets the window's location. Anything that doesn't run scripts never sees the hop.
HTTP redirects run first, then script redirects, then meta refresh. At each hop the browser sends the cookies it holds for that hop's own domain, and each server can set new ones. A chain can pass through a short link, a publisher's click tracker, a network and a sub-network before it reaches your page.
Most chains are ordinary. What matters is that each step is one your network can explain. The guide to reading an affiliate link follows one hop by hop.