A classifier, in the context of machine learning, is an algorithm that is trained to assign labels to instances based on their features. This process involves learning from a dataset that has examples already labeled (supervised learning), to make predictions about the class of unseen instances. Classifiers can be used for binary classification (distinguishing between two classes) or multi-class classification (distinguishing among more than two classes). Common algorithms used for classification include logistic regression, decision trees, support vector machines (SVMs), and neural networks. The effectiveness of a classifier is usually evaluated based on metrics such as accuracy, precision, recall, and F1 score.

The concept of classification has roots in statistics and has been part of computer science since at least the 1950s, with the development of the perceptron algorithm by Frank Rosenblatt in 1957 being one of the earliest examples. Since then, the field has evolved significantly, with the introduction of more complex algorithms and techniques.

Significant figures in the development of classification algorithms include Frank Rosenblatt for the perceptron, Vladimir Vapnik and Alexey Chervonenkis for the development of Support Vector Machines (SVMs), and many contributors in the area of neural networks, including Geoffrey Hinton, who played a crucial role in the development of deep learning techniques that have significantly improved classification performance.