CSS Units

Measurements

%
percentage ... the following is just to help visually understand line-height in percentages. The font-size is set at 100% and the line-height is set to 113%.
em
1em is equal to the current font size. 2em means 2 times the size of the current font. E.g., if an element is displayed with a font of 12 pt, then '2em' is 24 pt. The 'em' is a very useful unit in CSS, since it can adapt automatically to the font that the reader uses ... The font-size is set at 1em and the line-height is set to 1.125.
pt
point (1 pt is the same as 1/72 inch) ... the following is just to help visually understand line-height in points. The font-size is set at 12pt and the line-height is set to 13.5pt.
px
pixels (a dot on the computer screen) ... the following is just to help visually understand line-height in pixels. The font-size is set at 16px and the line-height is set to 18px.

Summary

In the end, it's really up to you which unit of measurement you wish to employ. Often times you'll see a mixture of units used. Pixels for font-size and or images, em for width values (such at the width of the #container) and font-size, the percent symbol (%) for width values and font-size. Only the "pt" value is normally limited to only font-size values. However, you'll notice, you'll need to change values depending upon the font-family you use.

For the summary: font-family: Arial, sans-serif, font-size: .9em, line-height: 1.25em