Opuntia argentina is one of the 'pad cactus', so-named because the leaves are, well...pad shaped. I could be wrong about the name, because the whole cactus family has a lot of variation within species, and the Opuntia genus is especially prone to look-alikes.
Opuntia argentina seems to be less cold-hardy than most Opuntia, losing small leaves and large-leaf margins in slightly sub-freezing temperatures. Like many other Opuntia, they look harmless enough, but if you touch them you quickly find that they have tiny, well-camoflauged spines in random spots. Not only that, but even if the leaf or stem looks woody and harmless, many Opuntia continuously grow new spines even in old growth. Handle with care.
Lots of food and water in the spring makes for a light green and rapidly growing plant. I just leave them alone in the winter. Give them as much light as you can. Opuntia are a tasty treat for mealy bug.
Bonus fact: mealy bugs are used to make the food coloring cochineal, which is used in many human foods such as Hi-C drinks. Yummy!