Tweak is right. A mode is starting on a different note in a scale. Evo is right too it would be C# dorian, the second modes in B major scale. i.e. B major, C# dorian, D# phrygian, etc. B dorian and B major are not the same scale though.
The second question, in rock you use one scale over the entire chord progression, in jazz style you use different scales over each chord. This is due to the way chord progression in each style. In jazz the chord modulate through sometimes several different keys so one key would not work over the entire chord progression. In rock however the chord progression for a passage tends to stay in the same key so you can use one scale over the chord progression. You may need to understand more theory to get what I mean but it should answer your basic question.