Visa bulletin is a monthly publication updated by the Department of State that controls the availability of immigrant visa numbers. It contains important information related to U.S. immigration. More specifically, the visa bulletin establishes cut-off dates for all employment-based and family-sponsored categories which are experiencing a shortage of visa numbers. Only applicants whose priority date is earlier than the cut-off date may apply for an immigrant visa or adjustment of status to become a permanent resident (receive a green card).
This page describes how visa bulletin works, including the concept of visa number "spill-over" which has been critical for employment-based immigration in recent years. You may also be interested in this easy-to-understand flow chart showing how cut-off dates are established each month by the Department of State's Visa Office.
Preference based immigration system sets an annual cap for the number of immigrants allowed each year, which ultimately determines the visa bulletin. However, the annual limits are not simple numbers; they are rather calculated based on complex formula set by various immigration laws. Here you will find a detailed explanation of how annual numerical limits and caps are established, for both employment-based and family sponsored categories, as well as green card lottery (diversity visa). You may also be interested in knowing how many green cards are actually issued per year, per preference category.
Visa retrogression is frankly the number one reason why most people are interested in visa bulletins. Unfortunately it has been quite severe for most EB2 and EB3 categories, and all family-based preferences. The retrogression tracker is an interactive chart that demonstrates the problem for EB-2 and EB-3, by displaying the wait time between cutoff dates and visa bulletin release dates for the past five years.