Ah, yeah. That would probably be helpful. When a person with a strong heart loses their heart in the universe of Kingdom Hearts, that person creates a Nobody very human in appearance. Under normal circumstances, they would appear just like their human selves and carry the memories of their human life, but they're basically just a shell of themselves held together by what we can only assume as darkness (considering that's what they spew when they're injured and what they fade into when they're ultimately killed). Ah, and they also do not carry emotion (they just replicate it through the memories they have).

Replicas, on the other hand, are more of an enigma. In Xion's case, she is essentially just a doll composed of what no ones really knows who only begins to showcase any kind of appearance or action after absorbing memories from Sora. And when she's shown to fade, instead of fading like a Nobody traditionally would, she gets to special snowflake it up with shiny little pieces of glitter crystals (man, check out this snowflake special. OH YEAH). There is also Riku Replica who I believe was another doll (again, the details are sketchy because, 'sup, Nomuraism) who had data from Riku's memories assimilated in him. Ah, it's important to note that the difference between the two is that Repliku had copies of Riku's memories while Xion Sora's actual memories. I don't even know.

Does that help clear things up?