Bank 1 relates to the bank of cylinders which includes number 1 cylinder, bearing in mind that a 'V' configuration engine has 2 banks of cylinders. So an in-line 4 cylinder engine only has "Bank 1".

Sensor 1 relates to the position of the sensor along the length of the exhaust. Sensor 1 being closed to the engine, always before the catalytic converter, while Sensor 2 (if fitted) is after the cat. Ive heard of Sensor 3, but yet to come across one...

So, your "Bank 1, Sensor 1" is the sensor closest to the motor.

Hope that helps.