I'm new to this game...just a few hours into a first play.

I'm cruising down this road, just trying to make it to the next town. Pretty fully loaded from stomping some bandits, so not trying to start trouble. Some Imp soldiers with a prisoner are going the other way. Some wolves get themselves killed (kinda funny watching the prisoner jump in there). I didn't assist, but took the pelts.

And these two gallumping dudes are off to the side of the road headed our way. The guards and prisoner regroup, ignoring these two, and head off down the road. I keep a wary eye on the two guys...who get closer...and bigger...and okay they are pretty big. Still got that gallumping gait. Don't seem inclined to bash me with big clubs...

And they just stroll on by. I stood in front of one and at that close distance noted that there's a ground shaking effect. Couldn't talk to them. They ignored me. I considered swatting one, but thought that might be remarkably stupid under the current circumstance.

Do these guys serve some function? Do they ever attack? How tough do I need to be to go after them?

That was just a random encounter with a strolling Giant. Sometimes it's two of them or one Giant and a cow. Those are docile, but the ones you come across at a camp will attack if you get too close.

Giants usually stick to their camps, but if you are attacked by one:
At a low level, try to flee. Usually you will be sent to the moon, but still try. Most of the time, they only attack if you trespass on their land.
At a high level, try to run under their arms as they swing their clubs. This leaves you at their backs, you you can get a good strike in before they turn around. If you are close to a city, run and let the guard handle him.

The ones in their marked territory can become hostile by loitering there or getting too close. The other Giants that just migrate tend to be peaceful unless physically provoked.

At lower levels, best not to test their patience, unless you are good at evading and finding a place to where they cannot hit your PC. At around level 20 (depending on type of build and spent points on health) you take can start taking them head on (just move out of the way from their attacks).

*damn, ninja'd*

Very cool. Considering my current level (3) I'm glad they weren't irritated by my curiosity.