Debian is a Linux OS, Maemo is the linux based OS that the n8*0 use. OS2008 is the newest updates for Maemo, diablo is the codename used for the os2008 release (as opposed to chinook which corresponded to OS2007.

the tablet's internet browsing is pretty good with flash and java support, Abiword and Openoffice (via easydebian) can edit documents, pidgin is a good IM client, and claws mail or the built in modest is good for emailing