Orca
The Orca is a Tau orbital dropship. It is entirely built for transport, and meant to operate in low-intensity combat zones; it has almost twice the cargo capacity of the comparably-sized Thunderhawk, while only mounting an underslung turret for last-resort self-defense or clearing a landing zone. Imperial Armour Volume Three mentioned "unconfirmed reports" of more heavily-armed variants, though no models or rules for these have appeared yet (though now that flyers are king in 6th edition, that may change some day).
A standard transport load of the Orca is twenty Fire Warriors (sitting in rows on either side of the passenger cabin), six Crisis Battlesuits (lined up in a crouching posture in the center of the passenger cabin attached to a ceiling rail to deploy them) and a dozen or so Gun Drones (attached to racks in the ceiling over the Fire Warrior's heads.) That is almost half a Hunter Cadre right there, and represents a good way of getting a small infiltration force behind the enemy's lines and extracting them again.
While the Manta has a far greater carrying capacity and is more heavily armed and armored than an Orca, the Mantas are more valuable and more limited in number. Thus the Orca is meant to fill in a smaller strategic gap of Tau deployment: it functions better for "flying quiet" to do stealthy insertions, or to muster quickly to swiftly redeploy Tau forces to react to enemy advances. Further, three Orcas offer a great deal more flexibility in deployment than a single Manta does.
The Orca debuted in the Warhammer 40,000: Fire Warrior videogame, and Forge World gave it a physical model and rules in Imperial Armour Volume Three.
Battlefleet Gothic
When creating Tau ships for Battlefleet Gothic, Games Workshop must have run out of fish names, because they gave the Tau an escort gunship called the "Orca" as well. They are too small and too packed with weapons to mount a proper gravitic drive, so they get towed along in the FTL "bubble" of larger capital ships.