A neural network is a simplified artificial brain similar to the human brain, though much less complex. In a nutshell, a neural network consists of neurons connected to one another, exchanging signals. For each connection, the signal level can be regulated and saved, enabling the neural network to learn. You can find an extensive documentation here. (German)

An audio signal is segmented and transformed to its frequency spectrum by means of a Fast Fourier Transformation and conducted to several neural networks specialised on a section of the signal.