Of the three types of trespass, only two are criminal offenses. The differences between them are mainly in the intent and knowledge of the person committing the offense. While different states have different wordings of their trespassing laws, the basic meaning and definitions are universal.
Simple trespass, the least severe and only non-criminal type of trespassing, is defined by the trespasser lacking knowledge of the fact that he entered private property unlawfully as well as lacking intent to perform unlawful acts once on the premises. First-degree trespassing is the most severe and is defined as knowingly entering private premises with the intent to commit a crime. Second-degree trespass is the crime of knowingly entering premises the person was not authorized to be on without the intent to commit a crime.
Second-degree trespass is considered a misdemeanor crime, the punishment for which is determined based on the nature and circumstances of the offense. This distinguishes it from simple trespass (which is not a crime) and first-degree trespass (which may be a felony).
Some simple examples of second-degree trespass would be entering a fenced-in area with "No Trespassing" or "Keep Out" signs clearly visible, and parking in a private parking lot without permission despite clear signs advertising its private nature.