Bitcoin Newbie Explaination Mining Now if the question is stupid forgive me.

From what i have understood so far( i could be wrong )


Bitcoin is a digital currency service. 


Q : What is difference between coinbase.com,blockchain.info and thousand other sites who give me a unique bitcoin address upon registration ? Also are those bitcoin address unique globally or per site ?


You can earn money by bitcoin mining ( letting a software solve some complex mathematical problems in your computer)


Q : why do we need to solve a problem ? cant it have like a database which checks id/password combination like paypal and others on a server ?
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.