Term Frequency  Inverse Document Frequency (TD/IDF)
Basic
Tutorial on Information Retrieval
Entry level university class tutorial that assumes some mathematical
familiarity, but which includes a fairly clear explanation of
TF/IDF.
Understanding
Inverse Document Frequency
Paper from Microsoft Research, Cambridge, authored by Stephen
Robertson. (PDF Document)
Using
TFIDF to Determine Word Relevance in Document Queries
Paper at Rutgers University, authored by Juan Ramos. (PDF  4
pages)
Query
Expansion for Short Queries by Mining User Logs
Paper examines short vs. long user search queries and discusses
expansion techniques, including tf*idf (Term Frequency & Inverse
Document Frequency.
From Microsoft Research.
Search
Engine White Papers, Research and Patents
