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.

Use "?" for missing attribute values. For more details see the page:

http://www.cs.waikato.ac.nz/~ml/weka/arff.html

Source: Weka FAQ