Jar is similar to zip files. In java , jar files contains sets of files (mainly class files). Can be used to bundle all the related project/module file together in compressed format. User can set the classpath to jar file and use it . In other words jar are bundle all the packages of a project/module in a single file and use it by setting the classpath to the jar file.
Executable JAR files can also be created. By 'executable' I mean that it's possible (and easy) to create a Java application all wrapped up inside a JAR file that a user (on any machine with a properly installed J2SE JVM) can simply double click to run. See Creating a JAR File for more information.