Bitcoin Newbie Explaination Mining
Q : What is difference between coinbase.com,blockchain.info and thousand other sites who give me a unique bitcoin address upon registration ? 


These websites are competing web wallet providers. You can also use a mobile wallet or a desktop wallet. bitcoin.org has a list of good options.


  Also are those bitcoin address unique globally or per site ?


Each generates their own addresses, but you can send money to someone who uses a different wallet provider by sending it to one of their addresses.