Ventricular fibrillation: is when multiple cardiac pacemaker cells in the ventricles are firing simultaneously in a disorganized manner, thus there is no organized contraction of the ventricles, which in turn leads to ineffective contraction of the ventricle, and lack of adequate circulation to the body. V-fib is ALWAYS pulseless (unless the person has a ventricular assist device, then they may present alert or with decreased mental status), the person is dead, although initially they may have seizure activity followed by a period of agonal breathing. They need unsyncronized defibrillation.
Ventricular tachycardia is when ONE cardiac pacemaker cell in the ventricles is firing rapidly allowing rapid repeated depolarization of the ventricles. This is and organized rhythm and some patients will retain adequate circulation, while others will present in cardiac arrest. In those with a pulse, as someone else mentioned, the treatment can range from IV Amiodarone or Lidocaine to synchronized cardioversion, it all depends on the person's level of consciousness and secondly their BP. I have had multiple patient's in v-tach that have been pulseless, one that was completely awake with a really good BP, and another one who was semi awake and a BP that was tanking. So the treatment varies as you can see. Obviously if they are pulseless its straight forward defibrillation. Just remember these patients can also present with brief initial seizure activity followed by agonal breathing.
Remember when it comes to your heart, the fastest wins. In other words whatever cardiac pacemaker cell or cells are "firing" at the fastest rate win, whether it is in your normal electrical conduction system or in another part of the heart. (this is key information to remember when trying to understand arrhythmias)