The Illegal Immigration Reform and Immigrant Responsibility Act of 1996 (IIRAIRA) strengthened U.S. immigration laws, adding penalties for undocumented immigrants who commit crimes while in the United States or who stay in the U.S. for statutorily defined periods of time.
The Act was designed to improve border control by imposing criminal penalties for racketeering, alien smuggling and the use or creation of fraudulent immigration-related documents and increasing interior enforcement by agencies charged with monitoring visa applications and visa abusers. The Act also allows for the deportation of undocumented immigrants who commit a misdemeanor or a felony.
The Act mandates that immigrants who are unlawfully present in the U.S. for 180 days but under 365 days must remain outside the United States for three years unless pardoned. If they remain in the United States for 365 days or more, they must stay outside the United States for ten years unless they obtain a waiver. However, if they return to the U.S. without the pardon, they must wait 10 years until they may apply for a waiver.
For more on IIRAIRA, see this Cornell Law Review article, this Journal on Migration and Human Security article, and this Center for Migration Studies article.
Illegal Immigration Reform and Immigrant Responsibility Act of 1996, http://www.uscis.gov/ilink/docView/PUBLAW/HTML/PUBLAW/0-0-0-10948.html