If you have a website or a blog with good traffic and subscribers, you can earn decent money by Affiliate Marketing. 

Many famous Online Shops/Companies like Amazon and Flipkart use Affiliate Marketing as a marketing technique to sell their products and to increase the sales of the regular sellers in their platform.

These companies pay you for selling their products. You get commissions, which is usually a percentage of the money received by selling the product.

Your sales are traced through your affiliate link. You will earn money when someone buys a product through your link. Your Affiliate ID is usually cached by the browsers and you earn each time someone buys a product, until the browser cache is cleared.

Once you receive your affiliate link, you can advertise through Social Media and other internet sources for advertising. 

It is advised NOT to share your affiliate link with your family and friends as Amazon usually doesn't pay for such transactions, if captured.

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