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.

You can read the draft chapter of the data mining book that describes Weka 3.0 to get familiar with Weka. http://prdownloads.sourceforge.net/weka/Tutorial.pdf

Source: Weka FAQ