The differences between render operator and viewport render are that the viewport render has:

## Blender Internal
- No [motion blur][1]
- No [panorama camera mode][2]
- No [anti-aliasing][3]
- No [Color management][4]

## Cycles
- No [motion blur][5]
* Uses [**preview** progressive samples setting][6].
- No [Color management][7] (will be in Blender 2.69)

## All
- No [image dithering][8].
- No [compositing][9].
- [Modifiers][10] and objects preview settings are used:
  - [Object render/preview visibility options][11].
  - [modifier][12] render/preview visibility options.
  - [subsurf][13] preview/render resolution.
  - [multires][14] preview/render resolution.
  - [curve][15]/[metaball][16] preview/render resolution
- No way to save output.

*Note: I'm not sure if this list is complete, these may not be the **only** differences.*


  [1]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Post_Process/Motion_Blur#Sampled_Motion_Blur
  [2]: http://wiki.blender.org/index.php/Doc:2.4/Manual/Render/Panoramic
  [3]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Options/Antialiasing
  [4]: http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.64/Color_Management
  [5]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Integrator#Motion_Blur
  [6]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Integrator#Sampling
  [7]: http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.64/Color_Management
  [8]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Post_Process/Edges#Dithering
  [9]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Composite_Nodes
  [10]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modifiers
  [11]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Data_System/The_Outliner#Toggling_object-level_restrictions
  [12]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modifiers/The_Stack#Interface
  [13]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modifiers/Generate/Subsurf
  [14]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modifiers/Generate/Multiresolution
  [15]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modeling/Curves#Curve_Properties
  [16]: http://wiki.blender.org/index.php/Doc:2.6/Manual/Modeling/Metas#Meta_Ball_Options