Was on my way back to Whiterun and a giant was standing in road. Is this part of a quest? If so, who do I talk to to start it?

The only giant I noticed standing in a road near Whiterun was being slaughtered by Aela/Vilkas/Farkas, sort of an introduction to the Companions, since they invite you along.

Edit: first time I assisted and they tried to kill me afterwards. The next time I left them to it and they complained I didn't help. No pleasing some people.

nah giants wander around, as well as have camps, kill him or dodge him depending on level.