Relationship between Accounts,Contacts In salesforce

Account and contact behaves as master detail in business logics but on UI it is a lookup relationship. Let me explain you a bit more.

You can create a contact without filling account i.e it shows that there is a lookup relationship between account and contact.

If you have created a contact with account and you delete that account then contact will be deleted, this shows that it is in Master-Detail relationship. This is a standard behavior.

So we can say it in both ways, but it documentation it is a lookup relationship.

For more reference you can visit : http://success.salesforce.com/ideaView?c=09a30000000D9y3&id=08730000000JL3KAAW

I hope it is helpful for you 

Thanks

Gaurav