Affidavit of Support
One of the major concerns that arise when people apply for longer term visas or immigration visas is that they don't become a burden upon the social support services once they arrive. America does not look kindly of aliens that wind up on welfare roles or end up receiving free medical attention from public hospitals. Unlike most of Europe, these services are not overly abundant in the United States and they are designed to help citizens only. With that concern in mind, some visas, particularly those that originate as petitions from a citizen or green card holder and student visas are looked at with an eye towards how the applicant is going to support themselves while in the country. For example a student wanting to attend university in the US has to prove that he or she can pay for the tuition and living expenses before a visa is issued. If they don't have significant resources their visa could be denied.

Affidavit of Support guarantees your financial responsibilities
If you are in the United States and have petitioned for a family member seeking a visa you can improve their chances of being approved by filing an Affidavit of Support. Basically you are stating that you agree to provide assistance to the visa seeker, either financially or otherwise (room and board) lessening the chances that the visa seeker will end up on the public roles. By providing these assurances you are in effect greasing the visa process and making it easier for the visa seeker. If you want to assist a family member and wish to provide an Affidavit of Support you have two forms to choose from.

Application for Affidavit of Support
The Affidavit for Support Form I-834 requires the most detailed and authenticated information regarding your financial capabilities. This form requires proof of employment as well as IRS certified copies of the previous year's income tax return. The Form I-134 can be filed with no charge and does not include as rigorous proof of financial standing as the I-834. However if you are not fully truthful regarding your finances, if you guesstimate some of the numbers, you could be charged with perjury and the person you supported could be deported. It's important to remember that this document is a legal agreement between you and the government. You are agreeing to support the visa seeker so they don't become a public charge. If you violate the agreement you are subject to fines and even criminal prosecution.