One the most important and fundamental laws of electronics is that
of ohms law.
This law defines the relationships between current, voltage and resistance.
A good way to understand ohms laws is an analogy with a domestic water
system.
The same way that an electric current flows through a copper wire, water
flows through a copper pipe in a water system.
Most people do not think to much about the fact but when they turn on
tap the water comes out as a result of the pressure that is provided
by a pumping station some miles away.
This water pressure is similar to that used to drive electricity around
a circuit, the potential difference or pd for short is the term that is
used  this is measured in volts.
This driving force is provided by a battery or in the case of mains
electricity a generator at the power station
Definition of 1 Volt
The unit of electromotive force or electric pressure. It is the electromotive
force(pd) which, if steadily applied to a circuit having a resistance of
one ohm, will produce a current of one ampere 
The water that flows through a copper pipe can be considered similar
to that which flows through an electric cable.
The current flow in a cable cannot of course come out of the cable like
water can leak out of pipes or come out of the tap
However a force is required to drive it and resistance to this flow
is encountered, the flow of current is measured in amps
Definition of 1 Amp
The standard unit of electric current. The current produced by a pressure
of one volt in a circuit having a resistance of one ohm 
An amp is quite a large measure of current flow and you will probably
find that most circuits will have milliamps flowing through them  that
is one thousands of an amp.
When water or an electric current flows then a resistance to that
flow is encountered, the measurement of resistance is an ohm
Definition of 1 Ohm
The unit by which electrical resistance is measured. One ohm is equal
to the current of one ampere which will flow when a voltage of one volt
is applied 
Now that you are aware of what a ohm, volt and amp are  I shall introduce
you to the mathematical relationship that is ohms law.
Definition of Ohms Law
the principle that the electric current passing through a conductor
is directly proportional to the potential difference across it, provided
that the temperature remains constant. The constant of proportionality
is the resistance of the conductor 
The definition above is quite a mouthful, it simply states that the
current passing through a conductor increases if you increase the voltage
Obvious really  increase the pressure (voltage) and you will find more
current will flow.
All circuits have a certain degree of opposition (resistance) to current
flow through them, the relationship is given by R=V/I,
An increase in current flow with the same voltage will assume an decrease
in resistance
The above triangle allows the student to calculate any of the 1 of the
3 variables if the other 2 are known.
Cover the variable that you require and perform the resulting calculation.
Consider the circuit shown below
The resistance of that circuit would be given by R=V/I 24/2 =
12 ohms
The current flowing through the circuit would be given by V/IR
24/12 = 2amps
The voltage would be given by I x R 2 x 12 = 24 volts
If you continue your studies to a more advanced level than GCSE then
you will find that ohms law is one of the most fundamental electronics
laws
