.jar and .war are both zipped archived files. Both can have the optional META-INF/MANIFEST.MF manifest file which hold informative information like versioning, and instructional attributes like classpath and main-class for the JVM that will execute it.
.war file - Web Application Archive intended to be execute inside a 'Servlet Container' and may include other jar files (at WEB-INF/lib directory) compiled classes (at WEB-INF/classes (servlet goes there too)) .jsp files images, files etc. All WAR content that is there in order to create a self-contained module.