[rohwer]

Translate IDF to "how uncommon is this word in the corpus?"

TF-IDF is acronym soup, but mathematically simple: IDF is a scalar applied to a term's frequency. And in the comparison, the numerator is the document overlap score and the denominator is the square root of the two documents. For more, Stanford's natural language processing course is the bee's knees: https://class.coursera.org/nlp/lecture/preview