Ammeters measure current, voltmeters measure voltage, and ohmmeters measure resistance.

These devices are often combined into one instrument called a multimeter or VOM (volt-ohm-milliemmeter).
Analog versions of ammeters and voltmeters typically utilize a galvanometer, which relies on magnetic effect.
A needle is deflected by an amount proportional to the current passes through the device (coil).
A good ammeter should have a resistance that is small compared to other resistance in the circuit.
A good voltmeter should have a large resistance.
This java applet shows how to utilize a galvanometer into a multimeter.
[b]The full scale current for the galvanometer is 50uA with internal resistance R=2k ohm.[/b]

If galvanometer was connected in serial with a 198kohm resistor, it becomes a 10V voltage meter.
Because total resisitor become 200k, it need 10 V between two ends to produce 50 uA current.
And the current flow will be proportional to voltage across the device.
If a smaller resistor were connected in parallel to galvanometer, it can be used to detect more current than 50uA.
For example: if R/9 were connected in parallel to galvanometer, then the total current flow through galvanometer is 1/10 total cureent flow through both elements (the whole device).

The following applet shows the simplified version circuit diagram for a real multimeter.

You can operate at different mode:
1. select the Mode (Voltage/Current/Resistance) :
2. select the appropriate range for that mode.
3. You can change values of the DC voltage source V and resistor R:

4. Hit the OFF/ON toggle button to connect/disconnect the meter to the test circuit.

5. Learn / Exercise Mode :
Cilck the mouse at the center of the meter  (within the white circle area)
will toggle between exercise and learn mode,

Learn mode: ( default )  
When the meter is connected to the test circuit, the needle of the meter will be shown.
Exercise mode:
The needle will not be shown, and you will have 3 chances to enter the correct value into the textField.