The Striker was a Colonial vessel which escaped the Fall of the Twelve Colonies and joined with the Fleet. It remained loyal to Commander William Adama during the short-lived schism between the military and civillian governments following the attempt on Adama's life by Cylon infiltrator Sharon "Boomer" Valerii (Kobol's Last Gleaming, Part II).
The Striker was damaged in a collision with a fuel tanker during this time due to the incompetent oversight of recently promoted Captain George Birch, CAG for Galactica in Lee Adama's absence (Home, Part I).
Note
This ship is of the same design as the Celestra from the Original Series.