Wires come in standard wire gauges (S.W.G.) where the smaller the diameter of the wire the bigger the SWG. e.g 22 swg has a smaller diameter than 16 swg.
Copper oxidises (tarnishes) making it difficult to solder. It is therefore tinned giving us 22 swg tinned copper wire, for example.

If two bare wires touch it forms a short circuit. To avoid this wires are INSULATED using PVC etc.
When wire are used for coils such as transformers the insulation is varnish. Since this is very thin wires will take the minimum of space.
A broken wire is called an OPEN CIRCUIT and prevents current flowing.

A single strand of wire 0.6 mm in diameter is called 1/0.6; this is very rigid and snaps if bent too often.
Flexible leads are made from several strands of wire. 7/0.2 is 7 strands of wire each 0.2 mm in diameter.
Fine strands of wire can be woven into a mesh or braid which can be used to screen out unwanted interference. (The sunglass effect).
Television aerial lead uses screened cable called COAXIAL CABLE.
Leads which carry small signals such as audio are often screened to reject external interference.

Where several leads are needed they can be combined into a single cable. This can be a multicore cable, a cableform or a cable loom.
Cables are often terminated in plugs or sockets which may take some time to connect. A quicker technique is to use INSULATION DISPLACEMENT CONNECTORS (IDC) which take only seconds to fit.

Thicker wires can carry higher currents than thin ones as bigger pipes can carry more water than thin ones.
 

7/0.2 can carry 1 amp maximum
16/0.2 can carry 3 amp maximum
24/0.2 can carry 5 amp maximum
32/0.2 can carry 10 amp maximum
 

Use wire strippers to remove insulation. Avoid nicking the wires or breaking strands in flexes.
When soldering avoid whiskers, burning insulation and wicking (allowing solder to run up under the insulation of flex which makes it rigid and brittle).

FIBRE OPTICS is often used instead wires in some applications.