A .jar file contains compiled code (*.class files) and other data/resources related to that code. It enables you to bundle multiple files into a single archive file. It also contains metadata. Since it is a zip file it is capable of compressing the data that you put into it. Couple of things i found useful. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.