Earlier quoted context omitted.
I have to say that it sounds like an excellent reason to drop gmail and move away from google products. I don't _want_ advertisers to know more about me.
Advertisers don't know more about you. Google matches advertisements targeted to demographic X with people that meet demographic X. It's like a stock exchange -- you say you want to sell 30 shares of AAPL, and you're anonymously matched with someone that wants to buy 30 shares of AAPL. Only the exchange knows who both parties to the transaction are, protecting you from evil transaction partners. Now, if you don't tru…
Only sort of. Let's say I'm doing general advertising but want to know the age and gender of people who click on my ads. So I place ads for "men age 18-20", "men age 21-24", "men age 25-35", ..., "women age 18-20", ... . I give each a different landing url. Then I as the advertiser learn more about my incoming users than I would without targeting. I can't pull all the demographic information the ad network knows because if the buckets are too small things like CTR estimation work poorly, but if I'm high volume I can get good resolution.