A person who willfully breaches the boundary of private property against the will and/or knowledge of the owner is trespassing. The same goes for someone who refuses to leave someone's premises when asked to do so. If someone must approach someone's residence to fulfill an official duty, it is with certain restriction. For example, person delivering mail may walk up the sidewalk but may not enter the front door. Furthermore, landowners cannot intrude on the residences of their tenants even though the property belongs to them.
In some states certain public places, such as outside school basketball courts, are at the disposition of the public outside of working hours. A person who unwittingly "trespasses" cannot be convicted of such unless the property in question has made clear a "keep off the premises" policy. In other states, an intruder may be held liable for trespassing on a public or private domain even if no damage to property has been done. For example, questionable doubt concerning the intention of intruders can be established at places suseptible to theft, such as construction sites. It is likewise forbidden to tunnel underneath or to invade the territory in the space above property.(For example, very low-flying planes flying overhead).
Check out the site below for more information concerning the legal definition of trespassing and how it was first applied in law.