Guitars typically have six strings. Each string has a different thickness. Starting from the thinnest string, the strings are called string 1, string 2, and so on, up until string 6. Strings 1 and 2 are called "plain strings" and are bare steel strings (unwound). Strings 3 through 6 are wound with metal. When holding a guitar, string 6 is the topmost string. Moving from up to down (i.e. from thicker to thinner) results in an increasingly higher pitch.

Source: Yamaha