A JAR (Java Archive) file is a package file format typically used to aggregate many Java class files and associated metadata and resources (text, images, etc.) into one file for distribution. JAR files are built on the ZIP file format and have a .jar file extension. They are used for distributing Java applications, libraries, and applets. JAR files can be executed directly if they contain a manifest file specifying the main class. This allows users to run the application without needing to extract the contents of the archive. JAR files also support digital signatures, which can be used to verify the authenticity and integrity of the contents. This is particularly important for distributing applications over the internet, as it helps to prevent malicious code from being injected into the application. Furthermore, JAR files can be compressed to reduce their size, making them easier to download and distribute. They are a fundamental part of the Java ecosystem, enabling modularity and reusability of code.