While data published in any format can be considered Open Data if associated with an Open Licence, the type of data format used can have significant implications for the usability of the data.
One way to measure the openness of the formats used is through the 5-star deployment scheme for Open Data. The greater the number of stars, the more reusable the data.
Under the Open Data Initiative, public bodies are asked to publish their data in the most open way possible and at a minimum 3 Star such as CSV, JSON or XML. Public bodies are also encouraged to publish datasets in multiple formats, for example, 1 Star (e.g. PDF), 2 Star (e.g. Microsoft Excel) in addition to the required 3 Star (e.g. CSV). 4 Star data means that the data uses Uniform Resource Indicators (URIs) to denote things and 5 Star data means that you link to other people's data. An example of a 4 and 5 Star data format is RDF (i.e. uses the Resource Description Framework. An example of 5 star linked data on the portal can be seen here.
An explanation of the different formats and how they can be used including links to some open source tools can be seen below.
Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. The design goals of XML emphasize simplicity, generality, and usability across the Internet. Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures such as those used in web services.