Bank 1 is the left (driver's side) bank. 2 is right (pass. side).
Sensor 1,2,3 are the order that the sensors are in from the engine.
Ex. - Bank 1, Sensor 2 would be the second sensor downstream from the motor in the left bank of exhaust.
There is a point of confusion though. If your exhaust goes from 2 pipes into 1 (y-pipe), then the O2 sesnors all become bank 1.
So, your exhaust may only have 1 O2 sensor in bank 2 (before the y-pipe), and you may have 3 O2 sesnors in the left bank (one before they-pipe, one after the y-pipe, and one after the catalytic converter).
Hope this helps.