the purpose and features of a relational database?

I have the impression that the term hierarchical is not clearly defined in this context. Microsoft uses the term in a manner that is consistent with relational data. I thought that hierarchy applies to related data but I was not aware of the use of the term in that context before a couple of days ago. Whatever the terminology, XML allows data to deviate from the rows and columns design that is basic to the relational design.

There are hundreds of books with answers to this question. There are many more sites in the internet with answers.

A few more points I want to add is that at the time that relational databases was invented over fourty years ago by Edgar Codd of IBM, there were many types of databases and the relational model was designed to be simple and powerful. Also, contrary to what many people say, XML is like a database format and as a database XML violates the relational model immensely. I will leave it for others to decide whether that is a good thing, but I think one reason XML is used so extensively is because the relational model does not work for everything.