I read through the forum and see terms like GSM/grps/GRPS, and am wondering if I understand correctly.

GSM is for phone?
GRPS is for email/text data?
grps is roaming?

By the way, what is meant by sending mail via SMS?

Am I way off?

You pretty much have it!

for our uses...

GSM = phone service
gprs = data services; non-BlackBerry
GPRS = data connection established for BlackBerry services

With SMS (Short Message Service), you'd send a text message to another phone number (whose service provides SMS capability). Different services provide different limits on the message length. Typically 100 to 180 characters per message, hence the name.

SMS is very popular outside of US (e.g. Europe and Asia)

Handy link shows message length limits: http://freesms.1888usa.com/

Feel free to check my:
BlackBerry Glossary [CLICK]

From my:
BlackBerry FAQ [CLICK]