Answer:
It is illegal to enter and pass through property that is conspicuously marked against trespassing, either with signs or DNR-compliant paint marks on trees or posts.
It is also considered trespassing to enter or cross through private property if told by the owner not to do so, or to stay on private property after being told to leave by the owner.
It is worth noting that these laws does not make it a crime for a person to unintentionally and innocently wander into private land that isn't marked at the boundaries in some way.
There are several other more obscure types of trespass concerning race track stables, railroads, farm land, and trespassing with voyeuristic intentions. See link, under Subtitle 4.