Opuntia is a very large genus of cacti, varying in size from 2in tall miniature plants to 100ft tall trees. They are native from Canada, to Chile and Argentina. The genus has been split several times, but there doesn't seem to be a consensus yet on the best way to do that. The name Opuntia comes from the name of a Greek city. All Opuntias not only have normal spines, but they also have thin barbed spines barely visible, named glochids. These can get stuck in your skin and be very irritating. In some species, they are particularly light and can become airborne when the Opuntia is moved. Wetting the plant before handling it might be a good prevention. Opuntias often have large, colorful flowers. The fruit is also large and generally edible. Incidentally the unripe fruit of the opuntia - when plucked from the mother plant and treated as above - is also capable of producing roots.