By default, Terminal uses the Bash shell which is pretty old and well documented so you might want to google that. A good way to start would be to learn cat, grep, sort, uniq, diff and learn how to pipe and redirect output.
There are a lot of commands you can install from the Terminal section in Cydia and I also suggest getting wget, awk, and maybe links and lynx (for grepping the -dump option) and ssh. If you have a terminal on your computer and set up ssh properly (see ssh in my sig), it can be the easiest and fastest way to transfer files.
Most of the commands are in the /bin and /usr/bin directories so you can scroll through there and poke around for interesting names.