Opuntia is just ONE type (genus) of cacti (cacti is a plural form of cactus).
Opuntia is also called "nopales". People eat nopales, especially Mexican. I ate once (fried with eggs and tomatoes), its taste was not bad.
2014-02-01 22:15:36 補充：
By the way, the berry from nopales is considered to be good for losing weight and antioxidant.
2014-02-01 23:10:34 補充：
It shall be cactus. After all, cactus is a more generalized term than opuntia.
2014-02-01 23:11:58 補充：
Unless, people do refer to nopales, then they would use opuntia. In fact, in US, you would hear nopales more than opuntia.