This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
What did the error message say? (Copy and paste it here, don't give us a vague description of it.)
How did you run the jar file? (Copy and paste what you typed at the command line.)
And to answer "what is the classpath" please just go out and find a Java classpath tutorial. Why should we take the time to explain something which every single Java programmer needs to know? There are tutorials out there.