OK -
1) Look at this tutorial:
2) Let's assume your entire program consists of one Eclipse project.
To create a .jar file, you'd generally use the Eclipse IDE: Project, <right-click>, Export, Export as .jar file
3) Let's say you want to export the .jar file manually, for whatever reason.
No problem - the tutorial above tells you how. Just:
3a) "cd" to the directory with your .class files
3b) Use Notepad (Windows) or vi (Linux) to create a "manifest.txt" file
EXAMPLE CONTENTS: Main-Class: MyClass
3c) Use "jar" to put the manifest and the .class files into a .jar file:
EXAMPLE: jar cvfm MyClass.jar manifest.txt *.class
4) You can create as many different .jar files from as many different projects as you want this way.
5) You would actually invoke your .jar file(s) as shown in DJ's reply.
Does that help?