You can preview and buy songs and albums directly within your iTunes library using Genius.
To hear 90-second previews of any of the recommended songs, click the small image of the album art to the left of the song name. The song will play and the icon will turn into a blue square. Simply click it again to stop the preview.
To buy the song or album, just click the price button next to the listing. You may be asked to sign into your iTunes account, but once that's done, your purchase will begin to download.
To view the iTunes Store listing for a song, album, or musician, simply click the text for the suggestion.