Term Frequency - Inverse Document Frequency
t: Term
d: Document
TF: Indicates how often a term appears in a document
$$ TF(t,d) = \frac{freq \ of \ t^{th} \ term \ in \ d^{th} \ doc}{\# \ terms \ in \ document \ d} $$
IDF: measures the importance of a term across a collection of documents
$$ IDF(t)=log(\frac{\# \ of \ docs}{\# \ of \ docs \ containing \ term \ t}) $$
$$ TF-IDF = IF(t,d) \times IDF(t) $$