Atrial fibrillation is a result of chaotic myocyte depolarization within the atria that sends electrical impulses past the AV node 300 to 600 times per minute. The rate of ventricular depolarization depends on how many of these triggers the AV node lets through and will be dependent on how healthy it is and what pharmacologic agents are effecting it. On the EKG there will be no distinct P waves but you will find fibrillatory waves (best seen inferiorly and in V1) which appear as a wavy baseline. Since the AV node randomly lets electrical impulses through, the RR interval will also be variable in atrial fibrillation.
See examples of atrial fibrillation