Open Data is defined as structured data that is machine-readable, freely shared, used and built on without restrictions. The Open Definition provides a more detailed definition of Open Data. To summarize the most important points:
Availability and Access: the data must be available as a whole and at no more than a reasonable reproduction cost, preferably by downloading over the internet. The data must also be available in a convenient and modifiable form.
Re-use and Redistribution: the data must be provided under terms that permit re-use and redistribution including the intermixing with other datasets.
Universal Participation: everyone must be able to use, re-use and redistribute. There should be no discrimination against fields of endeavor or against persons or groups. For example, 'non-commercial' restrictions that would prevent 'commercial' use, or restrictions of use for certain purposes (e.g. only in education), are not allowed.
Working with Data
If you want to know how to work with different data formats, view more (PDF).