Immigration fraud comes in many different forms. Lying on an immigration form submitted for yourself, entering into a sham marriage in order to help someone get into the country or get citizenship, or providing false information in order to help get someone else into the country through a sponsorship program are all examples of immigration fraud.
Some people are also accused of immigration fraud in connection with a false statement about their immigration status in order to get employment in the United States.
Immigration fraud can be charged as a felony, and if you are found guilty of immigration fraud, under certain circumstances you can be deported and never again allowed to return to the United States. If convicted you can also spend a significant amount of time in prison.