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A JAR file is just a ZIP file with Java inside. It bundles compiled .class files, resources (images, configs), and a manifest file (META-INF/MANIFEST.MF) into a single distributable archive. When someone says "download the JAR," they mean a self-contained Java package you can run or include as a library.
Executable JARs contain a Main-Class declaration in their manifest — run them with `java -jar app.jar`. Library JARs are added to a project's classpath so their classes are available for import. Maven Central, the world's largest Java package repository, distributes millions of JARs. If you've used any Java software, you've indirectly used thousands of JAR files.
Since JARs are ZIP files, you can rename .jar to .zip and extract with any archive tool to inspect the contents. Build tools (Maven, Gradle) manage JAR dependencies automatically. For deployment, "fat JARs" or "uber JARs" bundle an application with all its dependencies into a single executable JAR.