How does the keyboard provided by the MacBook Air models compare to the one used by the MacBook?

All MacBook Air models use a black "chiclet" MacBook-style keyboard with square-ish keys and it "feels" essentially the same way.

Like the MacBook Pro models -- and not the "non-Pro" MacBook models (with the exception of the "Unibody" MacBook "Core 2 Duo" 2.4 13") -- the keyboard provided by the original, "Late 2008," and "Mid-2009" MacBook Air models also includes backlighting and an ambient light sensor that automatically "measures ambient light and works with integrated software to adjust keyboard illumination and screen brightness based on user preferences".

The current "Mid-2011" MacBook Air models -- the MacBook Air "Core i5" 1.6 11-Inch (Mid-2011) and "Core i5" 1.7 13-Inch (Mid-2011) -- also have backlit keyboards. However, the previous generation "Late 2010" MacBook Air models -- the MacBook Air "Core 2 Duo" 1.4 11-Inch and "Core 2 Duo" 1.86 13-Inch -- do not have backlighting capability.

With physical access to the computer, it should be obvious whether or not the keyboard is backlit. However, for those attempting to purchase a MacBook Air online, EveryMac.com's Ultimate Mac Lookup feature makes it possible to lookup each model by a variety of identifiers and determine for sure whether or not a model has a backlit keyboard even without hands-on inspection.

What is the difference between the "multi-touch gesture" trackpad on the original, "Late 2008," and "Mid-2009" MacBook Air models and the trackpad on earlier Apple notebooks?

Apple explains that the MacBook Air introduces:

An oversize trackpad with multi-touch technology. You can pinch, swipe, or rotate to zoom in on text, advance through a photo album, or adjust an image. This gesture-based input so successful on iPhone and iPod touch now comes to MacBook.

This is perhaps best described in action:

Please note that this is modestly less capable than the "four finger, no button" mutli-touch trackpad used in "Unibody" MacBook and MacBook Pro models available at the same time.

What is the difference between the "inertial multitouch" trackpad on the "Late 2010" MacBook Air models and the trackpad on earlier MacBook Air lines?

The "inertial multitouch" trackpad introduced by the "Late 2010" MacBook Air models -- the MacBook Air "Core 2 Duo" 1.4 11-Inch and "Core 2 Duo" 1.86 13-Inch -- and used with subsequent models is more advanced than the "three-finger multitouch" trackpad on earlier MacBook Air models.

The trackpad on previous MacBook Air models supports "two-finger scrolling, pinch, rotate, swipe, tap, double-tap, and drag capabilities" whereas the "Late 2010" and "Mid-2011" MacBook Air trackpad supports "inertial scrolling, pinch, rotate, swipe, three-finger swipe, four-finger swipe, tap, double-tap, and drag capabilities."