Structured data is as the word says, data that is structured in a understandable way. Examples on this is data that is stored in a database. Here it is easy to get the infomation you want without parsing out any of the values you are looking for.
Semi structured data Edit
Semi structured data is data that isn't totally structured, examples on this is XML where the data can't be pulled out directly. The data is usually separated with tags.
Metdata is data that is used to describe other type of data and is used to describe the content of some data. Using metadata is a nice way to achieve interoperability. Different types of metadata is:
Descriptive metadata which describes a resource, example on this is title, author and abstract.
Structural metadata which indicates how objectes are connected to each other, example on this is how pages are ordered to form chapters.
Administrative metadata which is information that explains how to manage a resource, example on this is when and how it was created, ﬁletype and other technical information, who can access it.