What is a Guilds of Ravnica guild kit? Are they legal to use? What's special about them over other preconstructed product?
The guild kits from Guilds of Ravnica are preconstructed 60 card decks using cards for that guild, however the cards are not all standard legal. This is the third time Magic has visited Ravnica, and cards that were in older visits are also included in the guild kits, such as Tolsimir Wolfblood from Ravnica: City of Guilds, the first visit, or Boros Reckoner from Gatecrash in the second visit.
The cards in the guild kit have the guild's symbol as the set symbol, they are all updated to the current border frame style, including the new legendary style and all watermarked with the guild symbol. Even the basic lands are specially marked as from that guild kit, with symbolism in their text box watermark that matches the guild, but is not the common watermark for guild cards. The five main cards of the kits (one each) also are foil and have new art, these cards are Etrata, the Silencer, Niv-Mizzet, the Firemind, Izoni, Thousand-eyed, Aurelia, the Warleader and Trostani, Slesnya's Voice.