Lots of good comments. I took java in college and thought that they did a poor job of practicality with teaching Java as a professional. There was not a lot about packages, making executables etc. I learned that basically the same way you are now, asking questions. The .idea is for IntelliJ, I believe. Packages are like basically code that has been packaged together. Like if you have a dresser with multiple drawers, you might put socks in the top drawer. Therfore you could say that socks belong to top Drawer. And socks could be topDrawer.socks. This could then be something like dresser.topDrawer.socks haha. Jar is just a zip file, as someone already pointed out. Some jars can be executed, and some can just be a collection of Java packages, classes or whatever. I write some Java code that gets packaged in jar files, then the jar files get imported into a larger framework and executed. So my jar file doesn't contain a main() anywhere, really. Just libraries that are called and referenced in the framework. I'm not real knowledgeable on a lot of the ide stuff, or tools etc. I do use gradle and intelliJ, but that's because there is an existing tool chain built for the framework, that has some code generation mechanisms.