What is metadata?

Metadata simply means data about data, extra useful information on data - In context of databases, metadata would be info on tables, views, columns, arguments etc. For example, say you have a table to store personal info, which has columns that keep the social security number, phone, address, name, surname, birth date etc. for each person. While describing this table, you would also have to describe each column with a data type, and a length - precision etc. right? (The column to store the birth date of a person would probably be described as DATE type, whereas you might want to define the name column's data type as VARCHAR2- Here, what you call 'data', would be the dates which are stored in the table. The metadata would be all the other stuff you specified while you were creating the table, which defines how that data can be stored and accessed.  (Each RDBMS has its own way of storing and managing metadata I guess. All objects have extra info on what they are / how they can be accessed etc.)

I would like to provide an answer from the perspective of Performance Measurement.   Say we have a department with a number of performance measures floating around. There is a separate individual responsible for each of the measures. Whenever we need a number, we would approach the individual that is able to get the number for us.    Now to borrow from Russ Beinder's example - consider a library. With a lot of books inside. And a catalogue with a description of each book and where you need to go in the library to find each book.   The library in this case is equivalent to our department.   All of the books in the library are comparable to the different pieces of data floating around our department.   The library catalogue would be equivalent to the metadata, which is a catalogue of the DESCRIPTION of all the data in the department (i.e. what is the data, where can we find it, etc).   Take away point: Metadata is not a catalogue of the data. It is a catalogue of the description of the data.

From the "Metadata is information generated as you use technology, and its use has been the subject of controversy since http://www.guardian.co.uk/world/the-nsa-files was revealed. Examples include the date and time you called somebody or the location from which you last accessed your email. The data collected generally does not contain personal or content-specific details, but rather transactional information about the user, the device and activities taking place. In some cases you can limit the information that is collected – by turning off location services on your cell phone for instance – but many times you cannot."