A self-organizing map (SOM) or self-organizing feature map (SOFM) is a type of artificial neural network (ANN) that is trained using unsupervised learning to produce a low-dimensional (typically two-dimensional), discretized representation of the input space of the training samples, called a map.

http://www.mat.ucsb.edu/~g.legrady/academic/courses/06w259/projs/cs/MAT259-paper.pdf

http://en.wikipedia.org/wiki/Self-organizing_map

http://www.ai-junkie.com/ann/som/som1.html