Bank sensor 1 is the one closest to the engine, usually in the exhaust manifold. The lower ones are in the cat and are called bank 2. The upper (bank 1) o2 sensors usually have 1 wire coming out of them, the lower (bank 2) have 2 or sometimes 3 wires, as they are electrically heated. Be sure not to cut the wires on the car side of the connectors, as the new ones wont work properly when the wires are spliced. Do yourself a big favor, buy or borrow the special socket to remove them, as it makes them alot easier to remove without breaking.