A rooted cutting is a bit they cut off the main plant and rooted either in water or soil. For example for when I had to prune my philodendron, I cut of 4 long strands of stem. I then cut those 4 stems into sections that had at least 5 leaves on them. Then I stuck the stems in water where they produced roots which now means they are rooted cuttings. So it's not that big. They both can be referred to as a rooted plant however, but if I am correct a rooted plant probably means the full plant which should be in a pot.

Before buying you could contact the seller to clarify what they mean, and or ask for photos.

Rooted cuttings from a Christmas cactus would be just a few segments rooted. How much are they asking for them? I just found a huge pot of a true Christmas cactus for 25 dollars recently and was trying to figure out if it was worth it...it looks very old and they had several pots...so I'm still debating....

I'm sure each group (including growers of holiday cacti) has its own lingo, but I personally use "rooted cutting" in large part to signify its origin. A plant could be 20 years old and still be a rooted cutting. This distinguishes it from a seedling and several other possible origins. I also consider anything with roots, whether a cutting/seedling/etc. to be a "plant".