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.

Q: Is there a way I can add a dropdown list to a cell, so people can choose one item from a list of items located on another part of my spreadsheet or in a different sheet entirely?

It's actually pretty simple. Let's say you want a dropdown list in cell C2 that lets people choose from the items that are listed in cells H10:H25. Here's the steps:

1. Click in cell C2 and go to the Data menu and choose Validation.
   Make sure you are on the Settings tab and click on the dropdown that says "Allow" and choose "List".
2. In the box that says "Source", either type in =H10:H25 or simply click in H10 and drag to select H10 through H25 and the box will be filled in for you. (Or, if you don't want to use values from a range, you can just manually type in the values you want to appear in the list, separated by commas.)
3. Make sure there is a check in the box that says "In-cell dropdown"
4. Click OK.
5. Now check out cell C2. There's your dropdown list all set and ready to be used.