Italic, Latin, runic, and Cyrillic scripts are descendants from the Greek alphabet, which in turn was a creative adaptation of the Phoenician consonant script.

Read Wikipedia (etc.) about the relations.

Icelandic is a Scandinavian language, originally (and for a long time, and most notably) written in runes. When Latin script became a European standard, the thorn survived in many northern languages including English, until printers from the continent refused to cut matrices for it. Icelanders could not afford to have their language printed for quite a while, so they managed to keep their thorn.