What is the difference between GSM and WCDMA when switching to manual Network Mode? I was reading this answer and I need to comprehend this better:

What is the technical difference between "selecting GSM" and "selecting WCDMA" in Network Mode settings menu? To save battery life, I'm planning to switch from the automatic to the manual mode, and I want to know what happens if I stick to GSM or WCDMA.

I am on HTC HD2 with Android 2.2 FroyoStone.
What the phone's essentially asking you here is, do you want to use only 2G data networks, only 3G data networks, or automatically switch between them depending which is best?

The GSM setting will give you access to 2G (and 2.5G) data networks using GPRS or EDGE data standards and speeds.

WCDMA will give you access to 3G networks (using either normal 3G or HSDPA standards and speeds).

The normal setting would be Auto, which lets the phone automatically select the best (normally fastest) data network that it can pick up in the area that you're currently in, but as this involves using all the 2G and 3G chips and antennae this uses more power than forcing the phone to always use one or the other.

The downside to picking GSM is that it will stop your phone connecting to high speed 3G data networks, but will save power as the 3G (WCDMA) chips and antenna won't be used.

The downside to picking WCDMA is that it will stop your phone from using 2G networks at all (so if you're in an area with no 3G support, then your phone will have no data signal) but if you're in an area that you know has good 3G signal, then selecting this will save power.