The Crop is a block added by IndustrialCraft 2 used for cross-breeding. This block allows for plants to be harvested without breaking them. Crops gives the chance for two different plants to "breed" together to form into a better plant. You can plant Wheat, Watermelon, Pumpkin, Reeds, Carrots, Potatoes, Yellow Flowers, and Roses (Poppies in 1.7) in the crop block. A full list of crops can be found here.
The Crop is "planted" onto tilled, hydrated soil and allows for other seeds to be planted onto it. Empty crops will develop weeds that will spread to other crops and destroy planted seeds. A simple right-click will remove the weeds, but Weed-EX can also be used to stop a crop from being overrun by weeds. Be warned, as that will reduce the statistics of the plant that is growing on it. By placing Crops with one block with an empty Crop in-between, crossbreeding will be achieved.(X=crop with seed, x=crop with no seed. By planting two of the same or different seeds will crossbreed something different(Ex: X x X)).