1 Chapter, 14 verses, 299 words.
The author is John the apostle. In the first verses of both 2 John and 3 John the author identifies himself as "the elder". Note other similarities: "love in the truth" (v. 1 of both letters), "walking in the truth" (v. 4 of both letters) and the similar conclusions.
The letter was probably written about the same time as 1 and 2 John (A.D. 85-95). See the introduction to 1 John: Date
Occasion and purpose
See introduction to 2 John: Occasion and Purpose. Itinerant teachers sent out by John were rejected in one of the churches in the province of Asia by a dictatorial leader, Diotrephes, who even excommunicated members who showed hospitality to John's messengers. John wrote this letter to commend Gaius for supporting the teachers and indirectly, to warn Diotrephes.