[avemg]

No, that's not quite what it is. You can have an unusually rapid, but regular (i.e. normal sinus) rhythm and that's called tachycardia. During tachycardia, the electrical impulses that control the muscles of your heart fire in the correct sequence, intensity, and spacing between impulses, they just go through a cycle much more quickly. If you looked at an ECG and measured the distances between the peaks and valleys, they'd be at consistent distances, just really close.

During a-fib, the electrical impulses are all off. The peaks and valleys are irregularly spaced and the amplitudes vary from cycle to cycle. This usually results in a rapid heart rate as a second order effect because, due to the "misfiring" not enough blood is pumping through and your heart is working harder to make up for the difference.