On my acoustic, the first 4 strings are "rough" - that is they are covered in a type of metal corkscrew. The last 2 strings though are completely smooth, and don't have a corkscrew finish.
Why are the higher strings completely smooth? I do notice a slight difference in tone color (esp when the strings get older..) the smooth ones are brighter.
Why aren't they all smooth?
The terms are "wound" and "unwound" or "plain".
The larger gauge strings are a thin wire wrapped around a solid core. If a larger wire was simply used then it would be stiff and inflexible.
Take this cross-section. Now imagine it being bent in the middle. The wrapped blue wires move apart slightly as the string is bent and that's what makes it more flexible.
If the roughness of the wound string bother you then you may want to give flatwound strings a try. They look like this:
However, flatwound strings are considerably more mellow sounding than roundwound (normal) strings.
Here's a video by D'Addario showing how their strings are made:
The reason we have wound strings is due to the physics of the string's vibration. A heavier string vibrates more slowly, causing a lower pitch.
The wound strings could be solid wires, and achieve the pitch we need, however getting it to bend correctly across the bridge and nut, be easily fretted, AND be tunable, would be difficult. Imagine trying to do a blue's riff where you pull the low-E from the G on the third fret to A, with a solid wire. Imagine trying to tune that string; It would take pliers.
So, rather than use wire that should be used in fences, someone figured out that adding the mass as a wrapped wire around a thinner core wire would give us the best of both; a manageable string feel, with the desired frequencies.
When it comes to guitar strings, there are basically 2 main types:
Wound and plain.
Wound strings have a 'core' string of one material (usually steel), and have another length of metal string that is wound around that core. The most common material that the winding string is made of is usually phosphor bronze/plain bronze.
The winding produces the roughness that you stated. Round strings has two subdivisional types: roundwound and flatwound.
The surface of roundwound strings are bumpier because of the round edge of the string that winds around the core.
The surface of the flatwound strings are, um, flatter. This is because the edge of the winding string has a flatter edge, so when wound around the core, the surface of the final string is flatter.
Plain strings are just plain lengths of metal string, usually nickel or steel.
Wound strings are usually for the low E-G strings on an acoustic, E-D on an electric (although players who favour heavier gauge strings on their electric guitars sometimes have wound G strings)
Plain string are nearly always used on the high B and E strings, and usually the G string on an electric.
Because of the 'bumpiness' on roundwound strings, noise and string squeak can be produced when your fingers slide up and down them, particularly on the lower pitched strings. For this reason, faster playing, requiring more slide parts, is usually played on the higher strings.
When strings are new, they produce a sound that is usually described as 'crisp' and 'bright.' As they are played more, exposure to air, and the moisture and oil from fingetips cause the metal in the strings to oxidise. This coating of oxidation slightly dulls the tone of the strings, meaning they do not sound as clear as when they were new. I have come across some people that prefer that dulled tone however, so it is not necessarily a bad thing.
Hope this helps :)
