The video below shows an atrial flutter that occured due to valvular heart disease (the ventricular rate of this heart was controlled). Notice the rapid succession of identical, back-to-back atrial depolarizations.
On an ECG, an atrial flutter will have these identical 'irregularly regular' contractions with a characteristic SAWTOOTH appearance.
In contrast, atrial fibrillation gives an irregularly irregular heartbeat. There is no discrete/obvious P waves with irregularly spaced QRS complexes.