I'll try to expand a little for people who might not be familiar by adding my own overly simplified cardiology explanation.

Ventricular tabhycardia means that the heart is beating at a rate higher than the intrinsic firing rate of the pacemaker cells generating the impulse which tells the heart to beat, even though this isn't super fast, it's too fast for the heart when the impulse to fire originates in the ventricles.

In a normally functioning heart, the cells generating the impulse that cause cardiac contraction (pacemakers) are located in the SA node which has an intrinsic firing rate of about 60-100, which is why a normal heart rate is considered to be 60-100 for adults (the numbers are slightly different for pediatrics, but the concept remains the same.) When you have a heartbeat of 120, you have what is called Sinus Tachycardia because the pacemaker originates in the SA node and is above the intrinsic firing rate for that pacemaker.

If something goes wrong and the SA node is unable to fire correctly or consistently, the AV node begins to function as the back up "pacemaker" for the heart, but it only has an intrinsic firing rate of about 40-60. So, you can have what's called atrial tachycardia at a rate of 65. Because it originates from the AV node and is above the intrinsic firing rate.

If both of those go out, the heart keeps trying, and the ventricular cells become the "last resort" backup pacemaker, but they only have an intrinsic firing rate of about 20-40. So, while a heart is not beating "super fast" at a rate of 50, it's still too fast for an impulse generated by a ventricles pacemaker, and is called ventricular tachycardia.

As /u/43W1n said, if the patient is unstable, this rhythm is treated with defibrillation, which works by temporarily stopping the heart with the hope that when it starts back up, either the SA or AV node will have taken over as primary pacemaker and the patient will be in a more stable rhythm. (Defibrillation is a lot like turning the heart off and hoping when it comes back on again, the problem has gone away.) If the patient is stable, there are medications which might be able to resolve the situation without resorting to electrical intervention.

Ventricular fibrillation is uncoordinated electrical activity in the ventricles. While it's possible for someone in Ventricular tachycardia to be conscious and have a pulse, it is generally not likely with ventricular fibrillation (although there are some medical oddities and extreme cases).