There are two types of expectations of privacy:
A subjective expectation of privacy is an opinion of a person that a certain location or situation is private. These obviously vary greatly from person to person.
An objective, legitimate or reasonable expectation of privacy is an expectation of privacy generally recognized by society.
Examples of places where a person has a reasonable expectation of privacy are person's residence or hotel room and public places which have been specifically provided by businesses or the public sector to ensure privacy, such as public restrooms, private portions of jailhouses, or a phone booth.
In general, one cannot have a reasonable expectation of privacy in things held out to the public. A well-known example is that there are no privacy rights in garbage left for collection in a public place. Other examples include: pen registers that record the numbers dialed from particular telephones; conversations with others, though there could be a Sixth Amendment violation if the police send an individual to question a defendant who has already been formally charged; a person's physical characteristics, such as voice and handwriting; what is observed pursuant to aerial surveillance that is conducted in public navigable airspace not using equipment that unreasonably enhances the surveying government official's vision; anything in open fields (e.g. barn); smells that can be detected by the use of a drug-sniffing dog during a routine traffic stop, even if the government official did not have probable cause or reasonable suspicion to suspect that drugs were present in the defendant's vehicle; and paint scrapings on the outside of a vehicle.