.properties

.properties is a file extension for files mainly used in Java-related technologies to store the configurable parameters of an application. They can also be used for storing strings for Internationalization and localization; these are known as Property Resource Bundles.

Each parameter is stored as a pair of strings, one storing the name of the parameter (called the key), and the other storing the value.

Unlike many popular file formats, there is no RFC for .properties files and specification documents are not always clear, most likely due to the simplicity of the format.