Did you know that most typists use serif fonts (like Times Roman & Times New Roman) for text? Or that this type of font is designed so the reader's eye moves smoothly from letter to letter? Yep. The little squiggles you see on serifs are part of that ease-of-reading process.
Traditionally, typists use sans serif fonts (without squiggles) more often for headings, accounting, data entry, etc. These less decorative fonts also complement serif fonts when, for example, one is used for body text and the other for headings.
Today's home and office (ink jet & laser) printers usually operate at a minimum of 300dpi (dots per inch). At 300dpi and higher, both serif and sans serif fonts are readable, but on the Internet...
Most screen resolutions are set at 72-100dpi. Web typography experts often suggest that at this low resolution, Web designers offer the best readability by using sans serif fonts such as Arial, Helvetica, and Verdana. This will undoubtedly change in years to come, but slowly. Most users will not replace their current monitors simply because better resolution is available. Designers may be the biggest exception.
TIP: All fonts are not created equal. Verdana, an attractive sans serif font created for the Web, is slightly larger than Arial. If space on a page or in a heading is important, you may want to use Arial.
Incidentally, the text on a user's screen is almost always controlled by the fonts the user has on his/her system (hard drive). That's why most designers give a minimum of two choices when writing the font command. Some users, for various reasons, control the specific font and size their browser uses. Size is another, more difficult issue. You may learn that you can't please all the people all the time!!