overkill; i got carried away, but for future reference:
Pixel Density
PPI = pixels per inch
PPI is a pixel
density. it depends on the resolution
and screen size.
Example: a 16" by 10" screen with a resolution of 1440x900 has a PPI of 90. (1440/16 = 90 = 900/10!)
(Note: remember screen sizes are measured diagonally! 16" x 10" is a 18.8" screen; (Use Pythagorean Theorem!))
the iphone 4 has a lower resolution than an imac, but the number of pixels it displays per unit area is (much) higher, so it has a (much) smoother looking screen. it has a much higher ppi.
list of displays by pixel density
Size and Smoothness of Text
Example:
let's say a webpage is designed with 100-pixel-tall font. on a 100 PPI screen, that font will be 1 inch tall. on a 200 PPI screen the font will be 0.5 inches tall.
so, the higher the PPI, the smaller the font.
however, if you zoom in on the 200 PPI screen to double size (200%), the font is now 1-inch tall. but because there are twice as many pixels, font-smoothing can show smoother curves and sharper edges.
... Zooming
In Safari, for example, you can zoom in/out by pressing [apple] + [+/-], or simply by two-finger pinching on the current multi-touch trackpads.
In Safari, there are two ways to zoom; You can zoom the whole page, or opt to 'zoom text only'. If you zoom only the text, it can change the arrangement of the page elements and make things look awkward. If you zoom the whole page, you'll be zooming in on the images too, which are typically saved with compression for the web and will often reveal their low quality when resized. Either way, its a compromise.
Zooming is definitely very useful, and maybe necessary on some of the recent screens, but it does have its drawbacks.
11" Air vs 13" Air
as HH and AK pointed out, the pixel density is higher on the 11" (highest of any mac ever, it seems).
that means smaller text at 100% zoom, but zooming so that text is the same size on both,
the 11 will be more smooth and crisp.
the difference between PPI on the 13" Pro (113) and the 13" Air is noticeable in person, and that's about 7%. but it's true i've never noticed much of a difference between the Air screens, so i don't know;
i haven't done too many side by sides, i just know i like the ppi on the airs and hate it on the 13 pro.