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, but its only weaponry is 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.
A standard transport load of an Orca dropship 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 faaaar more heavily armed and armored than an Orca (it's more heavily armed than most things in 40k, actually), 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.
When creating Tau ships for Battlefleet Gothic, Games Workshop must have run out of aquatic names, because they gave the Tau an escort gunship called the "Orca" as well. They are too small to mount a proper gravitic drive, so they get towed along in the faster-than-light "bubble" of larger capital ships. Despite this, they are full of weapons that will cause big blue explosions on whatever they hit.
|Forces of the Tau|
|Command:||Cadre Fireblade - Ethereal - Ethereal Guard - Tau Commander|
|Troops:||Drone Squadron - Fire Warrior Team - Pathfinder Team - Stealthsuit Team|
|Auxiliaries:|| Great Knarloc - Gue'vesa - Kroot Carnivore Squad - Kroot Hound |
Krootox - Lesser Knarloc - Nicassar - Vespid Stingwing
|Battlesuits:|| Battlesuits (XV02 Pilot Battlesuit - XV15 Stealthsuit - XV22 Command Suit |
XV25 Stealthsuit - XV46 Vanguard Void Suit - XV-8 Crisis Battlesuit
XV88 Broadside Battlesuit - XV9 Hazard Battlesuit - XV95 Ghostkeel Battlesuit
XV104 Riptide Battlesuit - XV107 R'varna Battlesuit - XV109 Y'Vahra Battlesuit
KV128 Stormsurge Ballistic Suit - KX139 Ta'Unar Supremacy Armour)
|Vehicles:||Devilfish - Hammerhead - Piranha - Sky Ray - Tetra|
|Flyers:||Barracuda - Manta - Orca - Razorshark - Remora - Sun Shark - Tiger Shark|