Many cells have something "in" them -- they have a value. A spreadsheet gives you considerable control over what that value looks like.
Of course, most spreadsheets give you access to the usual word-processing-like formatting options: boldface, italics, centering -- that sort of thing.
More useful are tools given you to change how a value is viewed. Take the number 0.25. This can show up on the screen as:
...depending on what meaning you need to ascribe to it. The number is still the same to the computer no matter how you make it look on the screen; changing the formatting is strictly for the benefit of humans trying to make sense out of the data.
It's incredibly important to always make your data look the way it should. If you're dealing with currency, make the cell look like currency! Same with percentages! It might not make any difference to the computer, but a human reader (including you, if you come back to a spreadsheet after a couple weeks) won't be able to make heads nor tails out of your data if you don't format it correctly.
