What is the difference between Jhana and Samadhi?
Samadhi is a general term for concentration. Jhanas are stages you achieve in Samatha meditation when your concentration reaches a certain level. 

To give an analogy, Samadhi is like the temperature of ice. When the temperature reaches a certain level, it becomes water. That's like reaching the first Jhana. If you keep heating, it will become steam. That is like reaching the second Jhana and so on.