Wire, in its simplest form is a string of metal, that typically is cylindrical in shape. Wire can be square, cylindrical, or any of a number of shapes depending on it's purpose, but we will focus on basic, round wire here. The wire we speak of could be the electrical cable running through the walls of your home. For this discussion we will refer to wire in its use as an electrical conductor, or pathway. So wire is a string of metal connecting two things together.
If we think of wire as a connection of two things far apart, an easy analogy is then a street or a road. The road our wire provides allows electricity to pass from one place to another. Just as a road is rough, has ambient wind, and hills, this causes a bit of resistance to your drive. Wires too have an inherent amount of resistance, and this is the fundamental root of most wire discussions. It is important that the wire you use be able to handle the resistance for your system without getting hot, failing, or making things around it fail. Therefore, the most important electrical fundamental that will apply to this discussion is resistance.
What is resistance?
Resistance is a measure of electricity's reluctance to go through a conductive substance. Another way to say the same thing, resistance is the amount of drag on the electrical current passing through your wires. Either way you say it, the amount of power at one end of the wires is less than the amount of power put into the wires in the first place. This loss can be measured, mathematically by Ohm's Law. The OHM's Law discussion is here, but the short version is (current (I) = voltage (V) divided by resistance (R)). Resistance explains the loss of current due to the length of the wire and the diameter of the wire and conductivity of the metal being used in the wire.
So if the metal makes a difference, how is wire made?
Wire is made by a process called "drawing", the process simply "pushes" the raw metal through a hole in a series of dies, continually making it smaller and longer, or drawing the metal out. Depending on the gauge of wire desired the drawing will continue through progressively smaller and smaller holes in the drawing dies until it becomes thin enough to be bundled, or adequate size for a solid wire. Wire can come in a large variety of different formats and we will explain advantages of each in detail.
If this is your first go round, remember that typical differences in wire include but are not limited to the metal used, the gauge of the internal wire strands (aka is the wire solid or stranded), the diameter (gauge), what the wire is protected and covered in (insulated or jacketed). The longer the piece of wire you use, the more these differenceces will affect your installation.