>if its only function is to send you to some affiliate offer - it's spam //
I don't think this definition is quite right.
A site can be good and still have as its raison d'etre referring you to a product manufacturer. After all any sales website is doing the same thing, there's just a certain distance between affiliate sites and the manufacturer.
Don't get me wrong, a lot of affiliate sites are bogus, unhelpful and given unwarranted prevalence in SERPs; but that doesn't make them all bad. For example in the UK there are sites that offer money back on all purchases - they're affiliate sites that share the affiliate fee with the purchaser.
Monetizing your website with affiliate offers doesn't make it spam. Having no other purpose than to redirect a visitor to an offer - does. Especially if you use less than lilly white SEO techniques. Here's a nice way to put it:
Google believes that pure affiliate websites do not provide additional value for web users, especially if they are part of a program that distributes its content to several hundred affiliates. Because a search result could return multiple sites, all with the same content, they create a frustrating user experience.
I don't think this definition is quite right.
A site can be good and still have as its raison d'etre referring you to a product manufacturer. After all any sales website is doing the same thing, there's just a certain distance between affiliate sites and the manufacturer.
Don't get me wrong, a lot of affiliate sites are bogus, unhelpful and given unwarranted prevalence in SERPs; but that doesn't make them all bad. For example in the UK there are sites that offer money back on all purchases - they're affiliate sites that share the affiliate fee with the purchaser.