The short version: an editor helps you manage the wishlist, a guest just views it, and a reserver is a guest who claims a gift to give. Editors are invited by email; guests only need the link.
Editor (Contributor or Full Access)
Someone you invite by email to help manage the wishlist. When you invite someone, you pick one of three access levels — Full Access, Contributor, or Viewer.
A Contributor can view and add wishes, but can't edit or delete existing ones, or change settings.
Full Access can view, add, edit, and delete wishes, and fully manage wishlist settings.
You grant editing access by email invite in your wishlist's access settings. It doesn't depend on whether the wishlist is public or private. See "How do I invite another person to edit my wishlist?"
Guest (Viewer)
Anyone who opens your wishlist link. Viewer is also one of the access levels you can grant by email invite.
A guest can view the wishlist and reserve items, if reservations are enabled, but cannot add, edit, or delete wishes, and cannot change settings. For an Anyone with the link or Public wishlist they need nothing but the link — no account required. See "Can people see my wishlist without an account?"
Reserver (gifter)
A guest who reserves a gift to signal they'll give it.
Reserving locks the item so two people don't claim the same gift. It isn't buying, though — Wishpicks doesn't take payment, and a reservation doesn't fulfill the wish. See "How does gift reservation work?" By default reservers stay anonymous, and you won't even see that something was reserved unless you turn on Reveal reservations.
Why it works this way
A guest and a reserver are the same person at different moments — viewing first, then reserving. An editor is a fundamentally different role: it's about managing the list, not gifting from it.