Adsense referrals is an "affiliate" program from google that pays you when someone joins adsense through your link. They have 15 different products that you can refer right now. Just login to your adsense account and click on "referrals". Read all about it.
Also there is Referral 2.0, which brings you possibility to choose related to your site products, then place ads with that product on your website, and when someone clicks on that ad and buy that product you will receive pretty good money.
Yes earning with adsense refferals is crap.Really siple ads are good.Rarely someone sign up under you,or download the product. But still it works good on very high traffic pages.
Well, i can tell you that if you do it right you can make excellent income on referrals. IMHO, the Firefox referral is the easiest because it's a good product and it's free.. however you have to "sell" it, so if your site is about banana-boats this might be difficult. But if your site is even remotely related to technology, the internet, or anybody that USES the internet often, this can be a pretty easy 'sell' .. As for the referrals 2.0, you also have to 'sell' or endorse them.. you cant just expect to slap them on a page and start getting conversions.