Trespass is a wrongful invasion of the property of another. Any unauthorized entry upon the land of another person is a trespass, whether or not the owner suffers any harm from it.
If a landowner wants to sue someone for trespass, he has to prove some things:
The landowner does not have to prove that the trespasser purposefully intended to trespass; all he needs to do is prove that the trespasser consciously intended to do whatever he did that messed with (however small) the landowner's property.
The general trespass statutes criminalize the entering and remaining upon the land of another when he is not allowed, invited, or privileged to enter or stay there.
A person is guilty of criminal trespass if he knowingly enters or remains unlawfully in a dwelling or premises, or if he knowingly enters or remains unlawfully in a building or upon real property which is fenced or enclosed in a manner designed to exclude intruders.
A person commits criminal trespass if he knows that he does not have the owner’s effective consent to do so but enters or remains on property anyway. A common example is if hunters set up a deer blind on the back end of a farmer's 80 acre property. They know the farmer doesn't want people on his land, but the hunters do it anyway, thinking that they won't get caught. If they do, they can get charged with criminal trespass because they knew that they were on the farmer's property and that the farmer does not want them on it.