The document discusses the K-nearest neighbor (K-NN) classifier, a machine learning algorithm where data is classified based on its similarity to its nearest neighbors. K-NN is a lazy learning algorithm that assigns data points to the most common class among its K nearest neighbors. The value of K impacts the classification, with larger K values reducing noise but possibly oversmoothing boundaries. K-NN is simple, intuitive, and can handle non-linear decision boundaries, but has disadvantages such as computational expense and sensitivity to K value selection.