The transparent ones that phase in an out? Yes those are other players playing.
The white phantoms can be off anything between a few seconds, and a few minutes. You can sometimes see the phantom of a phantom you've summoned into your world. I noticed it while playing Demon's with my girlfriend, and again while in the forest in Dark Souls while we were cosplay-ganking as Ornstein and Smough.
wow that's really great you get DS for free. I've just got DkS on 360, and i wish I could play Demon souls but I don't think that'll ever happen. on the 360 at least, its still pretty active. I dont see white summons as often but there's regular pvp in forest, burg and dukes