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.
A loose definition of clustering could be ``the process of organizing objects into groups whose members are similar in some way''. A cluster is therefore a collection of objects which are ``similar'' between them and are ``dissimilar'' to the objects belonging to other clusters. http://www.elet.polimi.it/upload/matteucc/Clustering/tutorial_html//
Source: Weka FAQ