Twinrova is the boss of the Spirit Temple in Ocarina of Time, the final big fight before Link faces off against Gannon. Twinrova is the witch twins, Kotake and Koume, who use the powers of fire and ice (resp) against you. You see both of them at various points as you make your way through the temple, generally saying funny things at your expense and sending Iron Knuckles after you. They basically look like grasshoppers with robes on, and tend to act like old women (reasonable enough, as they are roughly 500 years old). There are also hints that they raised Gannondorf, or at least had an influence on him while he was living in the desert as the Gerudo king.
The setting for the final fight is a large room with a giant block of stone in the center, with smaller pillars on all four sides. After climbing up to the top of the block, the twins start to circle you (on their brooms), taking turns firing blasts in your direction. To beat them, you use newly acquired Mirror Shield to reflect each blast toward the other one (for example, reflecting a blast from Kotake toward Koume). You do this by Z targeting your 'target', and them moving into the path of the blast (before it hits the ground). Sometimes they will be on opposite sides of the room, making the shot impossible; in those cases, just steer clear of the beam and try for the next one.
After a while they get annoyed, and, to teach you a lesson, combine into a single form (Twinrova). Twinrova is more human-shaped, with vast tracks of land and way too much makeup. In each hand she has a wand, one fire and one ice. Each time your shield is hit, it stores up the magic that was used, and starts to glow, and you'll hear a pulsating noise. After it's been hit three times by the same magic (ie, 3 ice hits or 3 fire hits), it will sound like a fire alarm. At this point you can fire off a beam of your own, hitting Twinrova. She'll fall onto one of the pillars; use your long shot to get over to her (the best method is to Z target her and then use the hookshot), and proceed to hit her with something until she gets up. I prefer the hammer, because her boobs jiggle when you do it, but those with less perverted minds might as well use the Biggoron's Sword. After a while she'll start to mess with you, by firing two of one shot and then throwing in one of the other - you don't want to catch the third shot in this case, since you'll get hit by the blast and lose the stored up magic. Just dodge it and wait until she fires the next shot.
After you beat Twinrova, the twins show up again, with halos, and proceed to argue for a while before being sucked up into a light in the sky (heaven?) I think it's a pretty amusing scene, actually. Before leaving they swear that they will come back and haunt Link, leaving them open for a return in a later Zelda - I hope so, these two are my favorite Zelda bosses ever.
Both of the twins also make an appearance in Majora's Mask, where they run the potion shop and tour boat in the swamp†, and Servo5678 informs me that "Twinrova also appears in the two GBC games The Legend of Zelda: Oracle of Ages and The Legend of Zelda: Oracle of Seasons", where they go around making a mess and saying mean things to Link. However, you cannot fight them in a single player game.
† Thanks to malcster for reminding me that only one of them runs the potion shop, and that the other runs the tour boat.