Open Data is data made available to the public by individuals and organizations for anyone to use, share, and modify for any purpose, "subject, at most, to measures that preserve provenance and openness." (Open Definition 2.1)
Good open data:
- can be linked to, so that it can be easily shared and talked about
- is available in a standard, structured format, so that it can be easily processed
- has guaranteed availability and consistency over time, so that others can rely on it
- is traceable, through any processing, right back to where it originates, so others can work out whether to trust it
Principles from the Open Data Institute, What makes data open?
For a good explanation of what constitutes a standard, structured format, see the Primer on Machine Readability for Online Documents and Data from Data.gov.