Metadata is data about data. Metadata plays a number of important roles in computer forensics:
- It can provide corroborating information about the document data itself.
- It can reveal information that someone tried to hide, delete, or obscure.
- It can be used to automatically correlate documents from different sources.
Since metadata is fundamentally data, it suffers all of the data quality and pedigre issues as any other form of data. Nevertheless, because metadata isn't generally visible unless you use a special tool, more skill is required to alter or otherwise manipulate it.
Wikipedia has a nice entry on metadata.