They're dead people! After dying, a being's soul would be transferred to the afterlife, known as Hereafter, where it would be sorted out to it's final resting place. They have the same abilities they had when they were alive, but are confined to Hereafter, unable to leave.
Souls don't usually look like their living selves. Their appearances vary, some looking like a better, more idealized version of their living self, while others appear to be a completely different species or gender presentation.
Souls aren't very interesting, as they are literally just dead people. They may look different from how they were when they were alive, and can do the same things they did when they were alive, but other than that they don't have any magic dead person powers, and they're stuck in the Hereafter.