from the command line, to shutdown your computer type -
what you're doing here is telling your machine to shutdown for system halt (-h) immediately (now)...
alternatively, to reboot type -
where the -r means 'restart'..
have a look at the 'shutdown' manual page (type 'man shutdown' from the command prompt)...
if your computer doesn't actually 'power off' when you go for system halt, that's another issue - have a search on these forums. it's a relatively common problem.