JAR files (Java ARchive) allows aggregating many files into one, it is usually used to hold Java classes in a library.

WAR files (Web Application aRchive) stores XML, java classes, and JavaServer pages for Web Application purposes.

In Java, JAR (Java Archive) and WAR (Web Application Archive) files are both archive file formats, but they serve different purposes.

JAR (Java Archive):
- Purpose: JAR files are used to package and distribute Java applications or libraries. They can contain compiled Java classes, metadata, and resources (such as images or configuration files).
- Structure: JAR files use the .jar extension and are essentially ZIP archives with a specific structure. They can include a manifest file (META-INF/MANIFEST.MF) that contains metadata about the contents of the JAR.
- Usage: JAR files are commonly used for standalone applications or libraries that can be executed using the Java Virtual Machine (JVM).
WAR (Web Application Archive):
- Purpose: WAR files are specifically designed for packaging and deploying Java web applications. They include not only the compiled Java classes and resources but also web-related components such as HTML pages, JSP (JavaServer Pages) files, servlets, and configuration files.
- Structure: WAR files use the .war extension and also have a specific structure. They typically contain a WEB-INF directory, which holds configuration files and classes specific to the web application.
- Usage: WAR files are used to deploy Java web applications on web servers, such as Apache Tomcat or Jetty. The structure of a WAR file is designed to support the requirements of Java EE (Enterprise Edition) web applications.

In summary, while JAR files are general-purpose archive files for Java applications and libraries, WAR files are specialized for packaging and deploying Java web applications. The choice between using JAR or WAR depends on the type of Java project and its intended deployment environment.