A data could be considered as publicly available when it can be accessed without any permission or password by every individual (not just government officers) and there is no restrictions for the amount of photocopies can be made if the data is in the paper form.
The data is available online if it can be accessed through the Internet from an official source.
This question addresses whether the data is in a form that can be easily processed by the computer.
File types such as XLS, CSV, JSON, XML are considered as machine-readable, while PDF, or HTML are not.
This question addresses whether the data can be freely used, reused, and redistributed by everyone without any restrictions.