<TT>, the terminal text tag, causes the enclosed text to display in a fixed-width font. This is useful in creating, for example, a list of words in which length is important, or in drawing something like a magic square of numbers.
For anything much more complicated, the <PRE> tag is perferable. However, <TT> is far less likely to cause bad formatting in the page of cool and such places.
The effects of the <TT> tag are perhaps best seen when viewing symbols of vastly different width; consider "'w' and 'i'" as compared with "'w' and 'i'" in normal text.