JAR stands for the Java ARchive. Before discussing on JAR files let us discuss about ZIP file first. A ZIP file is file format which can store one or more files in the compressed format. ZIP files are most commonly used formats. JAR is also a compressed file format which normally contains the java class files (.class files) and a manifest file (metadata). Sometimes it may contain java source code files (.java files), images etc. For better understanding JAR files are ends with the .jar extension. Here is the list of all JAR tutorial:

How to create jar file in eclipse?
How to create jar file using ant?