Weka is a collection of machine learning algorithms for data mining tasks. The algorithms can either be applied directly to a dataset or called from your own Java code. Weka contains tools for data pre-processing, classification, regression, clustering, association rules, and visualization. It is also well-suited for developing new machine learning schemes.
DistributionClassifier ceased to exist a while back. There is now just the Classifier class. (Every Classifier is now a DistributionClassifier.)
Source: Weka FAQ