clarification in tf-idf formula According to the wiki-article, http://en.wikipedia.org/wiki/Tf%E2%80%93idf the term frequency of term $t$ in document $d$ is given as follows:
$$
tf(t,d) = 0.5 + \frac{0.5 \times f(t,d)}{\max{\{f(w,d): w~\epsilon~d\}}}
$$

This is the augmented way of computation. But what does $w$ stand for?
A document is modeled as a set $d$ of words.  Since we're maximizing $f(w,d)$ over all $w\in d$, this means that $w$ ranges over all possible words contained in $d$.

In the future, please check a textbook before asking.  We expect you to do some research on your own before asking, and this is a standard topic that's well-covered in standard explanations of tf-idf.