iTunes and Playlists

Hi, I just purchased some music using iTunes on my iPod Touch. When I connected the device to my computer, I noticed that the songs I purchased are now in a treeview in iTunes 10 for Windows called "Manually Added Songs". I'm wondering why they were placed there, and not in a purchased playlist, which I see as being one of the radio buttons. If I remove these songs from the treeview, how would I go about adding them again if I need to in future? Thanks. Yours Sincerely, Kelly Sapergia