Class files, we use them once you make a java file, it will turn into a class file.
Explaining the compiling
In your server every time you compiler the class file is all compiled information that goes into the class file if you make an errors lucky we have a compiler to make sure if we have any errors we can fix them such as when you make and error in compiling it don't go into your java file sense you messed up, it will tell you have to fix it this is why it shows you the errors in the compiler, you probably already know this, well everyone does pretty much know it unless your a newbie at this.
Explaining class files
Class files are the compiled information from the java file without java there is no class files for class needs to have a java file except when you make a .java file compile you will make a class file and you can delete the .java file if its a handler of sort and you'll have a plain old class file.
Explaining .java files/.class files
Make a java file put the information you need into it compile it with no errors and you will make a class file class files are compiled from the java file, the class file will appear and when you open it you wont see regular java information you'll see a bunch of letters and numbers bunched up, that is the compiled java in there with you cant read.