There are several varieties of wound strings, referred to by their winding type: roundwound, flatwound, halfwound and hexwound.

Roundwound
The simplest and most common strings are roundwound. They have a round core inside and round-winding wire, wound in a tightly around it. These strings are usually usually the least expensive and most common, as they are simple to manufacture. Roundwound strings have a surface profile (the "bumps" of the winding) that causes unpleasant "squeaking" sounds when the player's fingers slide over the strings.
The roundwound surface also causes fingerboard and frets to wear.

Flatwound
Flatwound strings also have a round core, but the winding wire has a rounded square cross-section. When tightly wound, such strings have a much shallow profile (in cross-section), thus making them more comfortable to play, less damaging for frets / fretboard and producing less squeak sounds from fingers sliding along the string. However, the sound of flatwound strings differs considerably from the sound of roundwound, being generally less bright.

Halfwound
Halfwound string (also known as ground wound or pressure wound) are somewhere between roundwound and flatwound. Round wire is first wound around a round core, then polished, ground or pressed until the outside is flattened.
Because the polishing process removes almost half of winding wire, manufacturers have to use heavier winding wire. This thicker winding wire lowers frequency of slide squeaking, making it less noticeable.

Hexwound
Hexwound strings have a hexagonal core and a tight winding that closely fits a hexagonal shape. It means that the winding cannot rotate around the core. Sometimes it is cited that hexagonal core improves tones due to closer bond between the core and the winding. A negative effect of a hex core is that relatively sharp hexagonal corners are less comfortable for fingers and wear down the fingerboard and fret wire even faster than roundwound strings. Hexwound strings are more commonly used on bass guitars).
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