I remember someone saying that Servants are a pool, Heroic Spirits are a lake, and the Grand Servants are an entire ocean. Or was it the Divine Spirits?
Quick summary:
Heroic Spirit: Everything, both real history from the individual's life and false things that people believe about them. This includes contradictory information. Also, a "full Heroic Spirit" isn't actually alive or anything, if you summon one it's just a ball of data that humans perceive as light. While their soul exists in some form on the Throne of Heroes, a Heroic Spirit as a whole is more a record that sometimes gets updated than anything else.
Servant: Using a Class Container as a framework to build around, specific parts of the Heroic Spirit are taken and used to make the Servant. This is what you see 99% of the time. This is why Servants can only use things relevant to their current class, since it needs to fit in the framework (and what a fame boost most impacts, usually gives them access to more things than would normally come with a Class). After a Servant dies, what happened to them is taken back to the Heroic Spirit on the Throne, which is where "updates" to the record can come from, since sometimes learning about what the Servant did leaves a big impact on the Heroic Spirit in some way.
Grand Servant: Literally just a normal Servant but better. Better stats, more magical energy, stronger/normally unusable Noble Phantasms, etc. These technically came first, and the original 7 classes we see in the stories are just an imitation of them. Only summoned to deal with specific threats known as Beasts, but they aren't relevant right now beyond that fact.
Divine Spirits: They are just Gods that either lost or never had a physical body. A god with a body > Divine Spirit. Often a Divine Spirit is stronger than a Servant, but edge cases such as the best Servants or the worst Divine Spirits (*cough* Stheno and Euryale, Medusa's sisters *cough*) can have a Servant match or surpass a Divine Spirit.
Weird edge cases like Hercules also exist, but since he started as a Human and ascended to Godhood later, both a Heroic Spirit and a Divine Spirit exist separately.
For Joe specifically, as a wraith he alone wouldn't have enough "weight" to be a Heroic Spirit normally, which is why instead he is the "face" for all the nameless craftsmen and blacksmiths throughout history and legend instead, since together there's enough "weight" to fill out a Class Container.
