What are cover crops?

I really had no idea about the concept of cover crops when I lived in the Mid-west. But out here in the Pacific Northwest one organic practice that was highly recommended to me is cover cropping after the fall harvest. Once the rainy season begins, soil gets compacted and many nutrients, such as nitrogen get washed out of the soil. The idea is to not leave bare ground during the rainy season. Roots keep air spaces and when you dig the plants into the ground a few weeks before you want to plant again, the nutrients return to your garden.

This is oneovernight example of rainfall: 2 1/4 inches! January 7, 2013.

My crimson clover was scattered around the empty spaces and around a few remaining veggies. I have this bed scheduled to be turned over March 1st.