Meters are used to measure the amount of many things such as voltage, amperage, resistance, and some are used as guages for gasoline and oil in your car. A meter is composed of a needle attached to a bobbin with a coil of wire wrapped around it. The bobbin is put in a magnetic field by means of a permanent magnet. When a current flows through the coil, a magnetic field is produced in the coil.
The diagram above shows how motor action works. When a conductor with a current flowing through it is put in a magnetic field, one end of the conductor's magnetic field will go in the same direction as the magnetic field, and they will add. On the opposite side, the magnetic fields will oppose one another, and cause a weakened magnetic field. The conductor will then move in the direction of the weaker magnetic field. This is called motor action, and is what causes the bobbin in a meter to move.
To find the direction of a magnetic field in a conductor, put your left hand around a conductor, and point your thumb in the direction of current flow (- to +). Your fingers will point in the direction of the magnetic field. This is called the left hand rule.
Meters that measure different things have different names. An ammeter measures current, a galvanometer current and current direction, a voltmeter voltage, and an ohmmeter resistance.