What do the <sup> and <sub> element do?

Answer 1

<sub> is used in H20 where 2 is below H.
<sup> is used in 10 to the power 5 where 4 is above 10.
You can see this in work in below link
http://w3schools.sinsixx.com/tags/tryit.asp@filename=tryhtml_sup.htm

Answer 2

The <sub> tag defines subscript text. Subscript text appears half a character below the normal line, and is sometimes rendered in a smaller font. Subscript text can be used for chemical formulas, like H2O.

The <sup> tag defines superscript text. Superscript text appears half a character above the normal line, and is sometimes rendered in a smaller font. Superscript text can be used for footnotes, like WWW