Open data is data that anyone can access, use and share.
Open data has to have a licence that says it is open data. Without a licence, the data can’t be reused. The licence might also say:
that people who use the data must credit whoever is publishing it (this is called attribution)
that people who mix the data with other data have to also release the results as open data (this is called share-alike)
For example, the Department for Education makes available open data about the performance of schools in England. The data is available as CSV and is available under the Open Government Licence, which only requires reusers to say that they got the data from the Department for Education.
These principles for open data are described in detail in the Open Definition.
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