A test for voltage would be a test for electrical potential similar to a test for fluid pressure, it is a measurement that is a difference between two points, Ground and Negative or Positive measured in Volts. This measurement is made as a "Tap' into the system with one probe with the other at the ground usually the negative battery teminal/engine/chassis ground.
A test for amperage is a test that is used to determine the quantity of electrical current flowing through a circuit. This is a function of Voltage divided by Resistance or pressure and friction.
Current measurement is taken in series with the load, you usually have to disconnect a circut and place your measuring device /meter by connecting the probes to either side of the broken circuit, the current flowing thruogh this arrangement is mesured by your meter and you get a corresponding readout. There are inductive coil sensors that can be clamped around a conductor carrying a flow of current, the current flowing through the conductor will induce a proportionate current into the coils of the sensor which is either connected to a meter or may have a meter built into it. Generally clamp-on or pass-through inductive coil sensors are less accurate the cheaper they are, accurate devices are expensive and smaller currents are more diffucult to detetct accuratly then large. Most Multi-Meters or DMM have the capacity to measure 10-20 amps directly probe to probe, higher current readings usually require the use of an external add-on shunt resistor or inductive sensor.
Generally in a diagnostic scheme you will test for known values first, the expected resistance of a device or the expected value of the electrical potential that is to normally applied to any given device. If the measured value or values are within expected specs then the net step would be to accertain whether the expected current was actually measurably flowing through any given circuit. This measure of current flow is a dynamic value that changes depending on the function of the electrical device and the work it is doing.
This is my own simplistic explaination that might help. Good Luck! It's really quite simple, until it gets complicated!!!!LOL