Twin Boys

So I recently found out that I am having twin boys .. The first ultrasound I had at 12 weeks I asked the lady if they were in the same sack but she said they wasn't .. They were in different sacks .. I don't know much about twins so im just wondering if they will be identical ? When I went for a 3D scan it honestly didn't look like my sons were in different sacks , they looked as if they were in the same .. I will check again at my next scan but I'm so curious .. Is it possible for me 2 have 2 boys and they won't be identical ? Especially if they are in different sacks ..