Cookie

A cookie is a small file that a website saves in a visitor's browser to remember something about them. In affiliate marketing, the important thing a cookie remembers is that you sent that visitor. When someone clicks your affiliate link, the merchant drops a cookie, and if that person buys within a set period, the sale is credited to you.

That set period is called the cookie window or cookie duration, and it varies a lot between programs.

Visitor clicks your affiliate link
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Merchant saves a cookie in their browser
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Visitor buys within the cookie window
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Sale is credited to you

Cookie length directly affects your earnings. A 24 hour cookie means you only get credit if the person buys that same day. A 30 or 60 day cookie means you still get paid if they think it over for weeks and come back to buy. Longer windows are better for you, so it is worth checking before you promote a program.

Cookies are not perfect. If a visitor clears their browser data, uses a different device, or clicks another affiliate's link before buying, you can lose the credit even though you did the work of sending them. This is a normal limitation of the model, not a scam, but it is a reason your reported clicks and your actual paid sales will never match exactly.

To see how tracking fits the whole model, read how affiliate marketing makes money.