Visa Bulletin provides schedule for Immigrant visas for family and employment based immigration. The US government establishes a number of categories for immigrant visas and a certain number of immigrant visas available for each category. As a result, there is a wait time for people in each category to get their immigrant visa, except for those who qualify as intimidate relatives of US citizens (spouses, parents and children under 21 years old).
Each month the Department of State updates Visa Bulletin with current state of the wait time.
You may view the Visa Bulletin for March 2014 here.