The meters which are connected in series with the circuit whose current is to be measured are called ammeters. The power loss in ammeter is I2Ra where Ra is ammeter resistance. To have low power loss, ammeter resistance must be very low.
The meters which are connected in parallel with the circuit whose voltage is to be measured are called voltmeters. The power loss in voltmeters is V2/Rv where Rv is voltmeter resistance. To have low power loss, voltmeter resistance must be very high.
The construction and working principle of both the meters is same. Both are basically current sensing devices but they have following differences :