There is a lot of misinformation and misunderstanding about undocumented immigrants. While many people believe it is a criminal act to be in the United States without proper documentation, it is not that simple. There are several considerations that can impact the criminal status of an immigrant.
The American Civil Liberties Union explains that government agencies may pursue criminal charges against you if you enter the country without following the proper protocol. However, being in the U.S. without documentation is not a criminal act.
Your presence in the U.S. is not criminal regardless of your immigration status. It is the act of entering the country without documentation that is illegal. Most immigrants, though, do not enter the country illegally. They enter with documentation and simply overstay their documents or do something that renders their documents void. Such actions do not constitute a crime.
In most cases, the simple act of deporting you is all that occurs. There are no potential criminal charges.
The only exception to the lack of criminal charges is if the court previously ejected you from the country and issued orders that barred you from reentry or if a court ordered you to leave the country and you failed to leave. In these situations, you could face a criminal conviction and prison time of up to two years.
You should note that these details explain federal law. Sometimes, states will seek their own methods to manage immigration, which may be in contradiction to federal law. It is important to understand laws at every level and the legality of such laws.