A soul is the character. It's what records your character's development, notably skills, that can then pass on into the next lifetime and is used to accelerate relearning of the skills you knew in a previous life.
A spark is the life. It's what buys your character the 12 months of play time over which they live, grow and die. This can be shorter than 12 months if you die and die often. Once you can no longer spirit walk to reach your body, your spark expires and life ends.