To elaborate on what BC posted, it could be that the left brake could be adjusted too tight or possibly have a hose going sour, alternatively, the right side could be adjusted too loose, or perhaps have a hydraualic issue like a stuck wheel cylinder. Most likely, if it is in the brakes, it's an adjustment issue.
Another problem that could be a possability would be worn suspension/steering components. Check the control arm bushings (upper and lower, especially the rearward ones) and the tie rods on the left side. If worn, any of those could cause a torque reaction on braking, causing a pull, usually accompanied with an audible "clunk" noise.
