[It] offers performance upgrades to speed up page loading, new navigation features and tab isolation so that if you hit a bad site only that tab closes and not the whole browser.
WebSlices will give users a way to keep updated about a particular item on a web page like stock prices, the weather or an eBay auction.
Accelerators let users access Web services like maps or translations in a small window without having to leave the page. via news.bbc.co.uk
This version will not run on Windows 7. Users will have to wait until the final version for that.
See also More Details about IE8
For an easy walkthrough, listen to the video below.