Humans and witches are definitely a separate species, and they can't interbreed (which will be a plot point in one book). Witch blood has a specific enzyme that is used to invoke certain types of spells - which means that a non-witch could invoke, but not create, an existing spell if they had a container of witch blood. Spells can be bound to objects such as charms or talismans, un-invoked.
However, there are four different types of magic - Earth magic, Ley-line magic, Wild (Elven) magic, and Demonic (black) magic.
Earth charms definitely take witch blood to invoke, but Ley-line magic requires a person be either a Witch, Demon, or Human, to draw on the Ley-line.
Update: I added Human to the list of those who can draw on Ley-lines, based on a quote I just found in The Good, the Bad, and the Undead. However (major spoiler):
! The book describes <PERSON> believing that both humans and witches can draw on Ley-lines, but then brings up that Humans only acquired the ability after Elves began interbreeding with Humans. 227
It's possible that only Humans with Elvish genes can draw power from Ley-lines, I don't know if it was ever definitively explained.
Update 2 - as noted, I was wrong that Humans can directly use Demonic magic - I was confusing it with Ley-line magic.
Demonic magic has two aspects, Dealing Directly with Demons (Dangerous), which Humans can do, and casting a Demonic-type spell (a mix of Ley-line and Earth magic, which only Demons can do). These usually require very unpleasant ingredients and rituals, and draw on power from the Demon's realm (the ever-after). They also generate a residue, called smut, that sticks on the caster's aura, and is visible to anyone who can see auras - sort of a "hi, I cast illegal spells" glowing neon sign. :)
Making a deal with a Demon, if the summoner winds up owing a long-term favor to the Demon (as opposed to making payment on the spot), means they will also have a demon-mark put somewhere on them, also generally detectable (and frowned upon).
Demonic magic is illegal, and most books of Demonic spells are destroyed when found.
As for some of the other categories:
* Warlocks are just witches with weaker abilities, not a separate species.
* Werewolves are humans, who have been infected or cursed.
* Vampires are humans who have been infected with a specific virus. 227