The difference is almost unnoticeable. To make equation authoring easier iWork and iBooks Authors equation editors are in math mode by default so it isnt necessary to add math mode commands to your equations. Nvspace height makes a vertical space.
Other L a T e X units can be used with this command.
The font type LaTeX uses in math mode is somewhat special since it is optimized for writing mathematical formulas. For example in anintegral the tradition is to put a small extra space between thefxand the dx here done with the command. To see a description of the align environment see Aligning equations with amsmath Open an. Use in vertical mode.