Ugg. Focus shift.Yeah, there's a term for that right? "Non-parfocal" but neither is the Canon... for what that's worth.
No, they are two different things.
Parfocal means a lens maintains focus while being zoomed, it's a very useful feature for shooting video (whereas with stills, it's generally easy to refocus after zooming). The Canon 24-105L is not parfocal (the 17-40, 16-35, and 70-200/2.8 non-IS are parfocal). Not sure on the Sigma, but I'd guess it's not parfocal.
Focus shift means the focus changes when the lens is stopped down. No problem shooting wide open, but if you stop the lens down to f/5.6 or f/8 with a close subject, that subject will likely not be in crisp focus with the Sigma 24-105. With more distant subjects, the effect is masked by the deeper DoF. The 50/1.2L is notorious for focus shift (people call it a 'backfocus problem' usually because of a lack of understanding the real issue).
I experienced some Sigma focus shift today in fact. I was a bit perplexed when I was using LV to manually focus the 50mm at f/4, and when I took the shot it was slightly off. The 24-105L didn't have this issue. Is this due to the fact that when focusing the aperture is wide open at f/1.4 on the Siggy and f/4 on the Canon? That is kinda useless then if using LV. Makes AF accuracy even more important.