Short introduction to Vector Space Model (VSM) In information retrieval or text mining, the term frequency - inverse document frequency also called tf-idf, is
And if this doesn't work, please google "wikileaks".
Wikileaks is developing an uncensorable system for untraceable mass document leaking and public analysis
N. Slonim, и N. Tishby. SIGIR '00: Proceedings of the 23rd annual international ACM SIGIR conference on Research and development in information retrieval, стр. 208--215. New York, NY, USA, ACM, (2000)