Prunes are dried plums. They are cleaned and processed before drying. They don't normally need to be cleaned.
Yes, they are different, but sort of the same. A plum is a purple fruit that grows on trees. Prunes are dried plums, the same thing with most of the moisture removed, though they are not really dry.
Yes, there is soluable fiber in Prune Juice. Excellent for the treatment of constipation.