What is the difference between a Flush Light and a Semi-Flush Light?

The main difference between flush and semi-flus lights are how close they are fitted to the ceiling. 

Flush Lights

A Flush mounted light is a light that is directly against the ceiling with no gap, allowing the light to face directly downward. 

Flush ceiling lights are particularly useful where the room has a restricted height, and having a lower hanging light may get in the way. 

Semi-Flush Lights

A semi-flush light is when the light is fitted but still has a small gap between the ceiling and the light, by leaving the small gap between the ceiling and the pendant it allows the light to hit the ceiling, this then offers different shades of light from the pendant to hit differently around the room.

More information regarding the Semi-Flush lighting can be found here.

Conclusion

So, the main difference between the two is the height of the lights; semi-flush are normally higher than flush lights, and also the proximity to the ceiling; flush are normally tight against the ceiling, whereas semi-flush protrude further into the room.