What are the differences between structured, semi-structured, and unstructured data?

Explain XML uses for semi-structured data

Explain why XML may be useful for the transfer of structured data from one database to another. Consider such aspects as to whether there is a standardized relational format for data transfer, existence of libraries, and human readability.

Can XML documents be queried like relational databases, and if so, how fast and convenient is the process expected to be?