RE: What is Account -> Relationship to Opportunity defines ?
well to explain that let start from the beginning,
so usually when a lead is converted to opportunity ->it create 3 things(account,contact,opportunity)
so opportunity can be created using an existing account are contact.(check contact u can find the same)
By creating opportunity using one of them u can relate to other.(if u see the account and contact they are in 1:N relationship).
opportunity need a contact or account else can be created from a lead(which internal going create an account related contact for the opportunity).
That is why u have the relationship for opportunity in the ribbon for both contact and account.
hope i made it clear .