A special effect in movies popularized by the movie "The Matrix". In the movie, time is slowed down so much that the viewer can see very fast moving objects such as bullets. Scientist used this technique to study other fast moving objects such as the famous milkdrop shot.
Bullet time in movies however uses cameras that record at speeds anywhere from 100FPS to 2000FPS and then digitally rendered to creat a desired effect (such as "wave trails" left by bullets).
As far as I know, the earliest non-scientific use of bullet time was in Korn's "Freak on a Leash" music video.
Bullet time allows you to see very fast moving objects.
Cel-Shading is a recently popularized form of graphics arts. Cel-Shading has been used in the animation field probably years before it was a possibility in the video game field. In fact, a lot of the modern day Japanese animation is cel-shaded at some point or another (such as a 360 spin scene where it's hard to draw each angle of a character).
Cel-Shading in the video game industry has only appeared in a few non-animation based games, such as Jet Grind Radio, Rez, and The Legend of Zelda: Wind Waker. There are two advantages to Cel-Shading:
1. Better FPS. A lot of the time there's a lack of complex texture. Though the lighting is very stressed, the GPU doesn't have to process a lot of high VRAM using textures.
2. More of a animated look. For cases when you want a cartoon looking scene rather than a realistic one, and when flat shading just doesn't look right.