Lens fogging can occur when you switch from one fairly extreme temperature to the next. This happens even more so in humid climates. The same basic thing can happen to your camera and lenses that you've seen happen to your glasses or car windshield- a fast fog or haze created by condensation. This condensation can not only appear on the surface of the glass, but also may develop on internal parts.

The above and below images were taken with a lens that fogged-
the camera and lens were taken from an air-conditioned interior, to a very hot exterior.

To prevent this: Put your equipment in an airtight plastic bag before taking it from one environment to another. Let the equipment gradually adjust to the new temperature and then remove from the bag. Do the same thing when going back to the original environment.

Why you want to take this preventative step: 1) It won't actually save you time to skip it. If your equipment fogs, it can take a while to defog. 2) The moisture from the condensation will penetrate into your equipment. This will cause fungus to grow. If the fungus is not removed in a timely manner, the fungus can etch the glass which will ruin the clarity of you shots for as long as you use that lens. Fungus may be difficult for the untrained eye to detect, and is sometimes costly to clean. The condensation can also cause rust and internal problems in your camera which will affect its functioning.