A GIS data format specifies how geospatial data is stored in a file (or multiple files) on disk. The format describes the logical structure used to store geospatial data within the file(s).
While we talk about storing information on disk, data formats can also be used to transmit geospatial information between computer systems. For example, a web service might provide map data on request, transmitting that data in a particular format.
A GIS data format will typically support: