A .properties file is a plain text file used to store configuration information for Java applications. It typically contains key-value pairs, where the keys represent configuration parameters and the values represent their corresponding settings. These files are often used to externalize configuration data, making it easier to modify application behavior without recompiling the code. The format is simple: each line usually represents a single property, with the key and value separated by an equals sign (=) or a colon (:). Comments are supported using the hash symbol (#) or exclamation mark (!). Properties files are commonly used for internationalization (i18n) and localization (l10n), allowing applications to adapt to different languages and regions by loading different properties files. They are also used to store database connection details, API keys, and other application-specific settings. The java.util.Properties class in the Java API provides methods for loading and accessing data from properties files. This makes them a convenient and widely used mechanism for managing application configuration in Java environments. The simplicity and portability of the format contribute to its popularity.