Using an item that belongs to someone else without his permission is considered stealing. This is the case even if you have intention to return it to the exact place and in the same condition that you found it. There is a dispute if this is a Torah prohibition or rabbinic.
Even using the item of a non-Jew or a minor without their permission is considered stealing. see above for lengthier discussion about stealing from non-Jews or from children.
According to many poskim, even if the owner subsequently consents and says he does not mind that the item was borrowed, since the borrower did not receive permission before he took the item, he is considered a thief. see Stealing from a Family Member or Close Friend for the Halacha if you know the person would consent. For example, if I would borrow my neighbor's rake that he left outside to rake my leaves without asking him, I would be considered a thief, even if when I inform the owner he does not mind.
This prohibition applies to land as well. For example if you would stand or walk through somebody else's property bearing a No Trespassing sign, or park in a private parking spot without the owner's permission, this would be considered stealing. Even if there is no sign, but common sense dictates that the owner would object, it may not be used without permission
Cutting across the lawn or backyard of another homeowner is considered stealing, as many would object to have someone do this because of the invasion of privacy or ruining the lawn. If he is certain the owner does not mind or sees that the owner has allowed the shortcut to become established on his property, then one may cut through.
If a private parking lot has a sign restricting parking to customers or the like, it is prohibited for others to park there. If it is evident that the owner needs the lot for his customers it is prohibited to park there even without the sign.
If a person parks in a spot, in a manner that the owner of the parking lot would not approve of, such as blocking the entrance or exit, it is considered an act of stealing.
It is prohibited to block a private driveway by parking or double parking in front of the driveway, but this is not considered stealing. If someone blocks your driveway, and you have tried to tell them not to park there and they continue to nevertheless, one is permitted to call the police to tow the car away.
It is not considered stealing to borrow an item that will surely not be damaged from use, that nobody objects when others borrow it. For example, since nobody minds when you use their hanger, sit on their chair, or wash your hands with their washing cup, it is not considered stealing to borrow it. However, if a significant minority would object, even if not the majority, it may not be used without permission. Certainly, if the owner is standing there and objects to your usage, it would be stealing to use it anyway. Additionally, if the borrower is aware of some reason that the owner might object, he may not use it without his permission.
If it is clear that the owner does not object, one may borrow the item without permission. For example, if in the past one regularly borrows a particular item, it indicates that the owner does not object, the person may use that type of item without asking permission.
If an item is being borrowed to perform a mitzva, we presume that the owner does not object if it doesn't cost him anything, as people are usually pleased to have others perform a mitzva with their possessions.