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

Blender Internal

• No motion blur (sampled)
• No panorama camera mode
• No anti-aliasing
• No Color management

Cycles

• No motion blur
• No tiled rendering.
• Uses preview progressive samples setting.
• Render layer settings will be ignored.
• Pause and resume
• Uses dynamic BVH by default.
  See Why is rendered viewport faster than rendering individual frames? and What does static/dynamic BVH do?

All

• No image dithering.
• No compositing.
• Modifiers and objects preview settings are used:
  • Object render/preview visibility options.
  • modifier render/preview visibility options.
  • subsurf preview/render resolution.
  • multires preview/render resolution.
  • curve/metaball preview/render resolution
• Particle display percentage is used.
• No way to save output (except for a screenshot).

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