The default shell is busybox, so a fairly basic shell. You need to install xterm, but that is quite easy. Just add the repositories at http://maemo.org/maemowiki/ApplicationRepositories .

for the 770 there was a package for bash avaiable, but i have not seen that yet for the n800. i expect it will be made soon.

The n800 may not be the bestt thing to learn on as all documentation is striped out. there are no man pages.