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

## Blender Internal
- No motion blur
- No panorama camera mode
- No anti-aliasing

## Cycles
- No motion blur
* Uses **preview** progressive sampling settings.

## All
- No image dithering.
- No compositing.
- Modifiers and objects with the eye icon activated are used/rendered instead of the modifiers and objects with the camera icon activated.

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