JAR == Java ARchive.
A package format that Java applications / applets are often distributed in.
"JAR file is a file format based on the popular ZIP file format and is used for
aggregating many files into one. A JAR file is essentially a zip file that
contains an optional META-INF directory. A JAR file can be created by the
command-line jar tool, or by using the java.util.jar API in the Java platform."
Since they are basically just PKZIP files you can look inside them with standard zip tools e.g. WinZip if you want to for some reason.