What is the difference between fibrillator and defibrillator?

Putting it into simple terms, a pacemaker helps the heart beat whereas a defibrillator stops the heart for a moment. Pacemakers generate electrical activity in the heart for patients with certain cardiac arrythmias. A defibrillator actually generates a shock so strong that it stops the heart's electrical activity. The heart (unlike most muscles) generates its own electrical impulse and can therefore hopefully restart itself after this big shock. Defibrillators are usually used for more serious cardiac arrythmias.
Yes, a very big difference:

• a defibrillator is used to stop a heart that is beating in a random rhythm without moving blood in a pattern called Ventricular Fibrillation, to do this the defibrillator applies a single large shock which stops the heart completely briefly then the heart (usually) self starts with a normal rhythm that moves blood as it should; if the heart restarts in the random rhythm it will be necessary to repeat the single large shock
• a pacemaker is used to keep a heart beating with a normal rhythm that sometimes "forgets" to beat by applying small periodic shocks all the time at the pace of the normal rhythm