Safehouses can be used for saving and storing cars. You can get them by completing missions (Grand Theft Auto III) or buying them (Grand Theft Auto San Andreas). They cannot be seen on the inside until GTA San Andreas.
Grand Theft Auto
In Grand Theft Auto they are saving buildings.
Grand Theft Auto 2
They are also saving buildings marked Jesus Saves.
Grand Theft Auto III
They are obtained by completing missions to get to other islands. They are used for saving and storing cars. To save you must step into the elevator.
Grand Theft Auto San Andreas
They are bought, all except 2 (Johnson's House and the Car Garage in San Fierro). In this game you can go inside of them and go in the differnt rooms. To save step onto the spinning floppy disc.