Basic DB Question

I have inherited a rather huge DB and some of the tables are related and some are not...my question is For a DB in SQL server 2000 to be totally relational isnt it imperative that all the tables are linked together by some sort of key of?For good design then yes but its not essential. Relational does idicate that tables are related. But it is more that they can be, they don't have to be. If the tables contain related data i.e. orders, customers etc then one would suggest that they should contain keys for the related tables.