What is the difference between class and id?
A class is a type ("flavor") of an element that can be used throughout a document (if you declare div.chocolate, you can have any number of div class="chocolate" in your document).
An ID is like a name for one specifice element. (declaring div#george, you can only have one div id="george")

Difference between DIV and SPAN?
DIVs are, theoretically, for DIVisions of content. A SPAN simply SPANS a set of content. One difference is in their structural role - a DIV is block-level, getting its own box in the page layout, while a SPAN is inline.

Which unit should I use, ems pts or %s?
ems and %s are relative to each other and thus most friendly onscreen in terms of letting the user resize teh text in his or her browser.
Px are absolute pixels measurements and thus fixed onscreen.
And then some folks argue that you should use pts on print stylesheets and print sheets only, because they're designed as a print sizing technique.