My 2001 Toyota Camry has error code P-0330 Knock Sensor 2 Circuit. What sensor is that?

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If it's a V6 the knock sensor may be in the valley between the cylinder heads. To get at it you'll need to remove the intake plenum and injectors. Most times I've had to do this the sensor is fine but mice have been living under the plenum and pissing on the wiring / connector, or chewing through the wiring. The piss corrodes the connection to the sensor so you have to buy a pigtail with the correct connector and solder it in.

A knock sensor is basically a special microphone that listens for signs of engine knocking due to things like over or under advanced ignition timing, fuel octane, high combustion temperatures, etc. When this sensor picks up on knocking caused by any of the aforementioned and others, it will send a voltage signal to the ECU telling it to adjust ignition timing to reduce or eliminate knocking.

 The knock sensor bank 2 is located "on the driver side of rear side of the engine to the right of the exhaust pipe just below the intake manifold".

On a 4 cylinder motor the knock sensor is on the firewall side of the block. Do a search on youtube for replacing it and keep in mind the wiring harness connector may also need to be replaced. 

You have at least one sensor (if not two) that detect engine knock & sends a signal to your PCM to retard the timing when it senses that knock.  I have never worked on a Toyota, but you could do a search on line to find out where it is located on your engine &/or search you tube to watch someone changing one.  As was earlier mentioned, you could also buy a Haynes or a Chiltons or a FSM which would give you instructions.  As an example, on my '02 GMC with a 5.3, it has two knock sensors & they are located underneath the intake manifold & are inserted in the lifter valley pan. If you do decide to change the sensor(s) yourself, change the wire harnesses at the same time. 

It will be screwed into the block somewhere. If you can't easily see it buy the Haynes manual for your car. The manual will pay for itself anyway.

Older vehicle, the knock sensor is going to be on the actual engine block, lower end of the engine block close to the oil pan where it meets the engine block. One wire. 

Newer vehicles might have the knock sensor actually inside the ECM/PCM.