Classifiers are general computational models for assigning a category to an input. The inputs may be vectors of features for the items
being classified or data about relationships among the items. The category is a domain-specific classification such as
malignant/benign for tumor classification, approve/reject for credit requests, or intruder/authorized for security checks. One way to
build a recommender system using a classifier is to use information about a product and a customer as the input, and to have the
output category represent how strongly to recommend the product to the customer. Classifiers may be implemented using many
different machine-learning strategies including rule induction, neural networks, and Bayesian networks. In each case, the classifier
is trained using a training set in which ground truth classifications are available. It can then be applied to classify new items for
which the ground truths are not available. If subsequent ground truths become available, the classifier may be retrained over time.