what is the disadvantage of using subdomains like .co.cc instead of .com , .org or something like that ?
Well other than .com and .org are top level domains which promotes professionalism, quality and longevity (less likely to go away tomorrow) google doesn't consider those non TLD domains as high priority there fore you may get low rankings.
that means if i build a same site with .com and the other with .co.cc ...........than the site with .com will have a higer ranking?
no not necessarily. Sure .com gets a little boost out of the gate but it doesn't really matter. With a little tim and effort you can rank just as high with the lesser known tld's
You are right Brandon.Add.On. Google always consider top level domains as important. But I have a doubt, why people with blogspot are ranking in top ten in google with having subdomain?
I don't see any co.cc ranking high for competitive keywords in google.com There must be a reason In my opinion Google don't like free co.cc subdomains because they're used to spam in 99% of the cases. Get a proper .com domain it's as cheap as $8 (make sure to use a discount coupon for GoDaddy or NameCheap).
Its better to use other domains say .co.uk, if you wanna your website to be ranked for a specific country. in this case its UK. So disadvantage of having .co.uk is that its not easy to get it ranked for US. Its an indication to SE.