Wires are method for conducting electrons from component A to part B.   Folks ask Adafruit: which wires should I choose for my project?  To make an informed decision, one must know how wire is constructed and made available to consumers.

Materials

Wire may be drawn from different conductive materials, most often metal.  Gold and silver are excellent conductors of electricity.  But exotic metals are often soft and expensive.  Most practical wires you may use are made mostly of copper and tin.

Solid verses Stranded Wire

A good number of wires you may use are fabricated as a solid cylinder of metal. These are called solid conductor wires.  Solid wires, tend to be hard to bend as they get bigger in size.  

Some smart person found that, like rope, if you make a larger wire out of smaller strands of wire twisted together, you get the benefits of a bigger wire while getting the benefits of bendability.

Why wouldn't one always use stranded wire?  Stranded wire can be harder to make connections on the ends - all those wire strands make it hard to put connectors on.

Wire Covers and Jackets

Anyone who has accidentally touched some metal to the wrong part of a circuit and gotten a spark has found out that it is very important to have circuit parts not touch where they are not supposed to.  The method of doing this with wires is to coat them in a sheath of insulating (non-conductive) material.  Older wires used paper or cotton, very hard to deal with.  Modern wires most often use some sort of plastic coating for most wire sizes.  Tiny wires used in transformers, relays, and other coils may coat the wire in a clear enamel so it looks like it is bare but it isn't.  

To connect to the ends of insulated wire, you must remove a bit of insulation from end.  This is called stripping the wire and should be done carefully to avoid cutting through the wire or slipping and cutting your hand.  Wire stripping tools make the job easier.  Even enamel wire must be stripped on the ends, often gentle scraping with a knife or sand paper will do the trick.  Once stripped, the wire is ready to connect to the proper point in your circuit.