A temperature inversion occurs when cold air is trapped beneath warmer air. The temperature of the air in the air mass near the ground is colder than the temperature in the air mass above. Inversions often occur when the cold air is trapped in a valley surrounded by high mountains. In the US, LA and Salt Lake City often have inversions that trap pollutants near the ground causing haze and smog. This article explains what happens in a typical inversion in Salt Lake City.
This image from China shows an inversion in the Sichuan Basin.
This image is a combination of advection fog trapped beneath a marine layer in the Great Lakes.
This image is an example of widespread valley fog in the western US.
This image is from a high pressure inversion over Virginia and North Carolina.
Since this post was written, industrial development in China has worsened the deleterious effects of the normal winter inversion. Earth Observatory has images showing the current inversion.