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.
Weka is open source software issued under the GNU General Public License.
Yes, it is possible to apply Weka to big data!
Data Mining with Weka is a 5 week MOOC, which was held first in late 2013. Check out the MOOC site for video lectures and details on how to enrol into this course and a new, advanced Weka course.