What is the difference between 'Packet Data' and 'Data Call'?

Here's from the N800 Manual:

To use a packet switched data service in a cellular network, such as GPRS, select Packet Data.

To use a circuit switched data service in a cellular network, such as HSCSD, select Data Call.

Due to the forum help, I can now connect to the web via my Cingular cell phone. That works fine. I am curious what these settings mean and do. I didn't touch them when I was going through the menus on the N800. Would I be better off switching from one to the other? Can you even do that? Any clarification is appreciated.

Thanks.

Data calls are typically billed per minute and fixed speeds (e.g. 14.4Kbps)

GPRS tends to be significantly faster, although can be variable / bursty and billed by the actual amount of data used, not the length of connection.

Thanks to GPRS it's now practical to stay connected to Instant Messengers, schedule regular checks for new mail etc. Also means I can do some browsing (low graphics ideally!) in peace, without haven't to worry about how long I've been connected.

Last data call I made was a number of years back, with my trusty Nokia Communicator 9000i, AKA "The Brick"