CTR refers to the Click-Through-Rate....a ratio of how many clicks your ads received to how many times they were displayed (impressions). If you have 3 ad units displayed per page, that counts as 3 impressions.
no again, its the total clicks in your entire adsense account (all pages and websites which you have put the adsense code)
it depends on how you display your advanced reports, you can choose and the CTR changes accordingly: page, ad unit, individual ad.