- ESSENTIAL. Arachne was a human weaver who challenged the goddess Athena to a weaving contest.
(The "who" clause is an essential part of the sentence; without that clause, the sentence does not convey its main idea.)
- NON-ESSENTIAL. Arachne, who was a talented weaver, challenged the goddess Athena to a weaving contest.
(The "who" clause provides useful information, but the clause is not essential to the sentence; if you leave it out, the sentence still conveys the main idea.)
Here you see Arachne's contest with Athena; visit the Bestiaria for more information.