The differences between render operator and viewport render are:

Blender Internal

• No motion blur
• No panorama camera mode
• No anti-aliasing

Cycles

• No motion blur

All

• No image dithering.
• No compositing.

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

The differences between render operator and viewport render are:

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.

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

Added item from @Dan the Man

The differences between render operator and viewport render are:

• No motion blur (since this relies on changing frames).
• No anti-aliasing (for blender internal at least).
• No compositing.
• No image dithering.
• No panorama camera mode.

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

The differences between render operator and viewport render are:

Blender Internal

• No motion blur
• No panorama camera mode
• No anti-aliasing

Cycles

• No motion blur

All

• No image dithering.
• No compositing.

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