Think of an Affiliate Site as helping a big company with customer acquisition and sales.
Just the way that as an author you'd be willing to pay people and sites to help you get sales, pretty much any company in the world is willing to pay you to help you get sales for them.
Companies like Amazon and WalMart know the Internet is Infinite.
They have an internal set of metrics
1) Each new customer costs us $50 to acquire. Note: Can be less or more, just meant to illustrate.
2) For each $50 worth of products we sell, around $10 is the cost in marketing and advertising.
3) Over lifetime, an average customers earns us $250.
So they figure out - What if we let normal everyday people and smaller companies and websites help us. We let them have $10 or $20 for each new customer or $5 or $10 per purchase.
It's cheaper than what it costs us. And they have an economic incentive.
If you have a small website, then easiest is Amazon as it seels a variety of products and someone buying something (kitchen sink. TV, ebook, movie, etc.) is highest.
As you grow bigger you can become an affiliate for a variety of ebook sites (Apple, B&N, Kobo, Amazon).
As you become even bigger you can start offering your users links to other things like electronic stores (Best Buy), bank accounts, etc.
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These rules: Actually, the ONLY place you are allowed by Amazon to put an affiliate link is on the website you specified when you signed up!
You can't tweet them, or put them on facebook. You definitely can't put them in emails/newsletters or in the back matter of your books.
If you do any of these things, you are risking your account.
They are meant to prevent some company from growing too big. For small companies and for small blogs Amazon doesn't care or check.
Amazon's concern around these rules is that some site inserts itself into the NORMAL chain of user finding out about Amazon and starts costing them money on users who would go to Amazon anyways.
It's also an easy way to avoid dependency on any one site.
