This article distinguishes between serif and sans-serif fonts and discusses when each is appropriate.
Consider the following characters. The first is set in Georgia, a lovely serif font. The second is set in Verdana, an easy-to-read sans-serif font.
Notice the small decorative flourishes at the ends of the strokes in the left character. These are called serif. The right character does not have these strokes and is said to be a sans-serif font. (Sans is the French word for without.)
Times New Roman is a commonly used serif font. Arial is a commonly used sans-serif font.
Use serif for printed work
Serif fonts are usually easier to read in printed works than sans-serif fonts.
This is because the serif make the individual letters more distinctive and easier for our brains to recognise quickly. Without the serif, the brain has to spend longer identifying the letter because the shape is less distinctive.
The commonly used convention for printed work is to use a serif font for the body of the work. A sans-serif font is often used for headings, table text and captions.
Use sans serif for online work
An important exception must be made for the web. Printed works generally have a resolution of at least 1,000 dots per inch; whereas, computer monitors are typically around 100 dots per inch. Even Apple's much vaunted retina display is only around 300 dots per inch — much lower than print.
This lower resolution can make small serif characters harder to read than the equivalent sans-serif characters because of their more complex shapes.
It follows that small on-screen text is better in a sans-serif font like Verdana or Arial.