Boarding torpedoes are (as the names suggests) a type of torpedo used by the Imperial Navy to board enemy ships. Due to their role they act more like spacecraft than a weapon of war, and thus fulfill a role not too dissimilar from Shark Assault Boats. On the flip side of the coin, however, boarding torpedoes are less maneuverable and armoured than assault boats; on the gripping hand, they are a smaller target and thus more difficult to intercept. Boarding torpedoes are seen in the new Space Hulk: Deathwing game where our fabulous Dark Angels embark on the heroic mission to cleanse a dastardly Space Hulk. Due to this, we may have the first canonical picture of the boarding torpedo, looking more akin to a giant, flying drill than anything else.
They are essentially giant Hades Breaching Drills in space. Puncturing the side armor and anyone else who is unfortunate enough to stand in front of the giant drill into a fine minced meat.
If it goes off course, however, you're fucked - you'll be sent screaming into the Void.
Battlefleet Gothic Armada
In Battlefleet Gothic: Armada, boarding torpedoes act as the smaller and nimbler cousin of the Shark Assault Boat. Space Marines are the primary users of these for some reason (maybe for tactical and surgical deep strikes?).
While they are meant to behave like smaller Assault Boats in theory, in practice there is almost no difference between a boarding torpedo and a Shark Assault Boat, except that torpedoes only work in a cone in front of the ship's torpedo tube.