Fonts, or letter shapes, fall into two main categories: Serif and Sans Serif.
Serif fonts have little "feet" at the bottom of letters, like Times New Roman:
Sans Serif fonts, like Arial, don't have "feet" at the bottom of letters:
For the main text of a document, you may want to use a serif font. Their "feet" lead your eye across the page, and make long passages easier to read.
You may want to use a sans serif font for headings. This makes a nice contrast:
Sans Serif Heading
The heading of this descriptive paragraph is in a sans serif font. See how it draws attention to this paragraph below it?