Generally speaking, if you own property in California, you are the only one with an interest in the land and the only one with a right to use it. However, it is possible for someone who does not own a particular piece of property to gain a nonpossessory interest in it allowing the use of the property despite not being an owner. According to FindLaw, the legal term for this nonpossessory interest is an easement.
Boundary lines between properties in California are not always as clear as they should be. As a result, you may have a neighbor who puts up a structure that exceeds the boundary line and winds up on your land, whether in whole or in part. The legal term for this is encroachment. Your neighbor may encroach on your property purposely, or it may be accidental due to a misunderstanding or miscalculation.