http://www.imperial-library.info/content/fight-six-911th-cow has the best description:
the Giants came from Old Atmora, up there across the Northern Ice back in the gone-to-twilight-now age of myth... and settled here in the Skyrim, and all along the mountain ranges of our coasts. (Yes, they are our true ancestors-- do not believe your aunt from the university-- and, yes, we were once as big as them-- as tall as THIS-- but that is another story)... [text lost]... and after [the Great Calamity] happened [the clan-things (peoples? tribes? Text seems to indicate mankind as a whole, though that is debateable)]... we were of a kind disrupted... and we Nords fell into fighting and drove our Giant-kin up unto the mountaintops [and we were a wicked-folk for many years]... [until all] things had changed forever. Once the Moot resumed [(unspecified) years later] things got back to a new semblance of normalcy and borders were redrawn and agreed with in beer-talk, and raidings of the merethlands took everyone's mind off old feuds, and pretty soon (well, not pretty soon but whatever) the Giants began to come down from the mountains again. And they were a bit different than we Nords remembered, or perhaps we had forgotten much, but they would not speak to us anymore-- they would only smile in their lazy way, stomp over, and take our stuff.
There is also http://www.imperial-library.info/content/ode-tundrastriders, which describes their daily life.