Crops are the result of planting an item in the crop block. The Crop is shown ingame as four wooden stilts that can only be placed on tilled dirt. You can plant Wheat, Watermelon, Pumpkin, Reeds, Yellow Flowers, and Roses in the crop block. Plants in a crop block cannot be grown instantly with bonemeal and will instead just speed up the process. Fertilizer can be used for a better effect of this. Seeds are also proven to be the same affect as bonemeal in wheat crops
Once a plant is fully grown it can be right-clicked to be harvested without destroying the plant. Left clicking the plant will break it and if fully grown, has a chance to drop a Seed Bag. If there is no plant in the crop block, weeds will grow, which will spread to adjacent plants and kill them. If an empty crop block is right-clicked while holding another crop block, it will turn into a crossbreeder, which cannot be planted in.
If there are at least two plants adjacent to the crossbreeder, it will combine their traits to make a hybrid plant.
If weeds are a problem you do not wish to deal with, then you can spray your crop blocks with Weed-EX to prevent them appearing. Be careful, however, putting too much Weed-EX can make your crops less efficent than they were
For some assistance while you're off mining, the Crop-Matron can be used to watch over your crops as they grow.
For more indepth info about the agriculture system, there is a section dedicated to it in the IC2 forums: Agriculture Engineering