Acoustic pickups are generally used to help ease tuning string instruments, like guitars, without having external noises picked up by the tuner. This allows for a more precise tuning of the instrument.
This review is on the Neewer Clip-On Acoustic Guitar Pickup. The pickup clips right onto my guitar, and thanks to its rubber pads, it provides a secure grip without it marring the finish. There are several places where it can be clipped on. For example, it can be clipped on top of the neck right beside the tuning pins, or on the edge of the sound hole. Then, all you have to do is plug it into the specified input on your tuner.
This pickup can also be used to amplify your instrument. I have successfully amplified by guitar and ukulele by simply attaching the clip to the sound hole, and plugging the 1/4 inch plug into my amp. This method can also be used with other string instruments, as well as brass instruments.
All in all I think that this clip on pickup that I received from Neewer, at a discounted rate in exchange for my honest review, is a reliable way to help tune my guitar.
This clip on pickup can be found here on Amazon: http://amzn.to/1R2blfr