Choosing guitar strings is a very important decision. After all, this is the junction between your hands and your instrument.
Guitar strings come in two forms: unwound (plain) or wound.
Unwound strings are your High E (1st), B (2nd) and G (3rd) strings*. These are a solid strand of metal and they feel smooth to the touch. Unwound strings are suited for higher pitches.
Wound strings are your Low E (6th), A (5th) and D (4th). They are plain strings with metal wrapped around them to produce lower pitches. You can see the difference between wound and unwound, or simply slide the edge of your pick against the strings. Wound strings will produce a jagged, scraping sound.