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

The MacBook Air uses a "chiclet" black MacBook style keyboard with square-ish keys and "feels" 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 MacBook Air 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".

What is the difference between the "multi-touch gesture" trackpad and the trackpad used in 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 the "Unibody" MacBook and MacBook Pro models.

For the curious, the always excellent iFixit also discovered that the MacBook Air uses a "Broadcom BCM5974 touch screen controller chip... [and] this is the same chip you'll find in the iPhone and iPod touch."