Apache Mahout is an open source machine learning library that provides algorithms for recommendation, classification, clustering and other machine learning techniques. It started as a sub-project of Apache Lucene in 2008 and became a top-level Apache project in 2010. Mahout implements many popular machine learning algorithms like naive bayes classification, k-means clustering, and uses the Apache Hadoop framework to provide scalability in distributed environments. Major companies like Facebook, LinkedIn, and Yahoo use Mahout for applications such as recommendations, user modeling, and pattern mining.