Note that the rhythm is regular - each beat is an equal distance from the next beat, which means that the time between each beat is the same. The rhythm strip is the long tracing on the bottom of the ECG which shows several continuous seconds of the heart. This permits observation of the rhythm of the heart, and determination of the heart rate.
This ECG shows a heart in atrial fibrillation The most obvious difference is the absence of the P wave. Leads I, II, and III show the classical appearance of AF, the "undulating baseline". The tracing never really "sits still". The distance between each QRS complex in the rhythm strip illustrates that the heart rhythm is irregular. There is no pattern to the irregularity, so the rhythm of AF is called "irregularly irregular".
This ECG illustrated a heart in atrial flutter. This has very distinct appearance. The "flutter waves" that we expect to see in atrial flutter are noticeable throughout the ECG, but are very easy to see in the rhythm strip. The rhythm is still regular, although this is not always the case in AFL