Wild Crops can be found scattered around the world, growing in small patches. They can be harvested for food and seeds, which can then be cultivated themselves in the not-wild form.
Harvesting wild crops can be done with your fists, or with a Knife or other sharp tool. When broken, they will drop Seeds and some Products.
Multiblock
An example of a wild crop, in this case Wheat.
There are many different types of wild crop - every crop that can be cultivated has a wild variant that can be found in the world somewhere. See the list of Crops for all different crops that can be grown. Wild crops will look similar to their cultivated counterparts, but are more hidden within the grass. Wild crops will spawn in climates near where the crop itself can be cultivated, so if looking for a specific crop, look in the climate where the crop can be cultivated.
Multiblock
All different varieties of wild crop