Metadata is data about data. This refers to not the data itself, but rather to any information that describes some aspect of the data. Everything from the title, information on how data fit together (e.g., which page goes before which other page), when and by whom the data was created, and lists of web pages visited by people, can be classified as metadata.
Where is metadata stored?
Metadata can be stored in a variety of places. Where the metadata relates to databases, the data is often stored in tables and fields within the database. Sometimes the metadata exists in a specialist document or database designed to store such data, called a data dictionary or metadata repository. There are some types of specialist data files that include both the raw data and the metadata (e.g., the SPSS .sav data file and .mdd data file, Triple S .sss). More generally, metadata can be stored anywhere (e.g., in emails, questionnaires, data collection instructions, or spreadsheets).
Metadata and data analysis
In the context of data analysis, metadata has a more specific meaning. Metadata is the information that is required for somebody to understand how to interpret and use the data. The most common types of metadata that are useful for data analysis are: value labels and missing value codes (the domain), variable labels, variable types, relationships to other data, variable sets, and, in the case of surveys, change logs, weights, strata, and clusters.