This website has the information you're looking for.
When you sign the Affidavit of Support, you accept legal responsibility for financially supporting the sponsored immigrant(s) until they become U.S. citizens or can be credited with 40 quarters of work. Any joint sponsors or household members whose income is used to meet the minimum income requirements are also legally responsible for financially supporting the sponsored immigrant. If the immigrant receives any "means-tested public benefits," you are responsible for repaying the cost of those benefits to the agency that provided them. If you do not repay the debt, the agency can sue you in court to get the money owed. When in doubt, ask the benefit provider whether the benefit is a "means-tested public benefit."
As far as I know, you are NOT responsible for paying back personal loans, car loans or mortgage loans that they may take out. It is only a way to insure that the immigrant won't be on the 'public dole'.
Additionally - http://www.visalaw.com/01oct2/12oct201.html
A provision of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996 created a new Affidavit of Support, Form I-864, which creates a legal obligation on behalf of the person who signs it. This legal obligation means that the sponsored immigrant, the federal government or any state government can sue the sponsor if the sponsor fails to support the immigrant. The Affidavit is enforceable either until the immigrant naturalizes or has worked for ten years.
In the event the primary sponsor does not earn enough, a co-sponsor can be used. This is done on Form I-864A. The co-sponsor must be a family member of the primary sponsor by birth, marriage, or adoption. The co-sponsor becomes legally obligated to provide the same support as the primary sponsor and the obligation does not end until the immigrant’s naturalization or until the immigrant has worked for ten years.
For complete information, I'd recommend going to http://www.visajourney.com/faq/k1faq.htm
. A LOT of people have received help through this website - they have a lot of people there who know what they're talking about.