Wire is sized by AWG or BS gauge number. Larger numbers represent smaller wire. Some wire is classified as solid, because it contains one strand per conductor. Other wire is called stranded, because it consists of many strands per conductor. Stranded wire is far more flexible than solid wire.
Most wire is made from drawn copper.
Some wire is sold that is claimed to be made with a process that produces oxygen-free copper. Oxygen-free copper has a different metallurgical structure than common copper and may or may not conduct current better. Also recommended on a budget is Sound King wire, a 12 gauge oxygen free copper stranded cable. Scientifically, thinner wire has more resistance than fatter wire, so fatter wire will have less resistance-related effects. Resistance effects can be eliminated by using at least 12 gauge wire, particularly for long runs.