Current
Current is the flow of electrons in an electric circuit. Flowing water is
a good analogy of electricity. When water flows through a pipe, or down a
stream, there is current.
Sometimes the current flows faster, and sometimes it flows slower. If we were to measure how fast the current was flowing in a pipe, we might say it was so many gallons per minute. When we measure how much current is flowing through a wire, it is based on the number of electrons flowing past that point in one second. There is a unit of measure called the Coulomb that enables us measure the amount of charge an object has (e.g. an electron). Since there are billions upon billions of electrons flowing through the wires, we instead measure the charge with the Coulomb, which is 6,240,000,000,000,000,000 (6.24 billion-billion) electrons.
When one Coulomb of electrons passes through a wire in one second, that is one Ampere of current. Ampere is the basic unit of electric current. It is sometimes referred to as amps. When writing down a value of current, it is usually abbreviated with an "A" (e.g. 1 A = 1 Ampere).
Since we aren't able to "see" electrons or Coulombs of electrons, how do we tell how much current is flowing through a circuit? We use an ammeter to measure electric current.
Voltage
Water flows through a pipe because of water pressure. Water pressure forces
the water to flow. Likewise, electromotive force (EMF) is the pressure that
forces electrons to flow through a circuit. Electromotive force is also known
as voltage. The basic unit of electromotive force is the Volt. 1 Volt could
be abbreviated as 1 V.
If you wanted to measure how much voltage a circuit or battery had, you would use a voltmeter.
In your house, you have wires in the walls that carry electricity to lights and plugins. The voltage in those circuits (if you live in the U.S.) is about 120 V.
Likewise, in your typical American automobile, there is a battery that runs the electrical systems. The voltage of that battery is about 12 V.
Resistance
In the same fasion that only so much water can flow down or stream, or through
a pipe, only so much current can flow in a circuit. Water is limited by the
amount of friction it encounters as it flows. Electricity is limited by the
amount of resistance it meets as it passes through a circuit. However, if
we increased the water pressure in a pipe, more water would flow. If we turned
up the voltage, then more current would also flow. Resistance limits the current
that flows through a circuit for a particular applied voltage.
The basic unit of resistance is the Ohm. 1 Ohm could be written
as 
(greek
letter Omega).
In order to measure the amount of resistance in a circuit, you would use an ohmmeter.
Conductors and Insulators
There are some materials that electricity flows through easily. These materials
are called conductors. Most conductors are metals.Three good electrical conductors
are gold, silver, and aluminum.
Insulators are materials that do not let electricity flow through them. Four good insulators are glass, air, plastic, and porcelain.