Apothecary
An Apothecary is a Space Marine specialist in Warhammer 40,000, combining the roles of medic and geneticist. The senior Apothecary is known as the Master of the Apothecarion (the fortress-monastery's medical center) or Chief Apothecary. They are usually armed with standard Space Marine gear and a Narthecium.
Since Space Marines have undergone genetic augmentations to survive nearly anything, an Apothecary's real battlefield roles involve three jobs: stabilization, mercy killing, and extraction. While Space Marines can endure damn near anything except the most serious injuries, Apothecaries frequently need to stabilize them long enough for their gene-seed to start up and heal them. If a Marine has been too badly injured to be saved, then the Apothecary applies "the Emperor's Peace," a mercy kill by driving a Reductor (a sharpened needle) into the Marine's skull to quickly end his pain (although some Apothecaries use bolt pistols, for some reason). If a Marine has died, it is the Apothecary's duty to extract the progenoids from his dead brother. Off the battlefield, they also implant Space Marine Initiates with the extracted progenoids, and make sure Battle-Brothers don't have any nasty mutations.
That's not to suggest they don't actually know any medicine, by the way.
Fun fact: back in the days of Rogue Trader, they actually were called Medics for a while, but that quickly changed.
Sanguinary Priest
The Blood Angels are a little weird, so they call their Apothecaries "Sanguinary Priests." There's a lot of weird blood rituals they do.
Also, they LITERALLY carry the blood of Sanguinius (How much blood Sanguinus would have before being completely drained from his already dead-beyond-dead body is unknown, although cell cloning of his blood might be a reasonable theory), since they're injected with a sample of it when they become Sanguinary Priests. This means they are the prettiest Marines, as they look the most like Sanguinius.
They also share a role with the Blood Angels' Chaplains in the spiritual guidance of their Battle Brothers. Where Chaplains are there to save Blood Angels from falling to the Black Rage, Sanguinary Priests watch for signs of the Red Thirst, doing their best to curb its effects, or when the battle turns against them - harness it as a weapon against the enemy.
Sanguinary Priests also oversee the induction of new recruits into the chapter, by a process known as Insanguination. During the Insanguination, geneseed is implanted into successful Neophytes by drinking from the Blood Chalice held by a Sanguinary Priest. Because of this role, the Sanguinary Priests are responsible for the care of the Red Grail, the chalice holding the blood of Sanguinius himself. This blood is regularly circulated through the veins of the Sanguinary Priests, who are injected with a sample from the Grail, then later cut themselves to bleed it back in. In this way, the blood is continuously kept fresh, and every single draught from the Grail contains a small percentage of the original blood of the Primarch....yeah we told you that it is a weird ass ritual.
Sanguinary Priests are armed almost the same as a regular Apothecary but they also wield a Chainsword and a Blood Chalice which is the chalice containing Hawk Boy's pretty little blood. Although how the Priest can go into battle effectively with what fields as practically a giant metal cup is unknown (Bonking the heads of Heretics maybe?).
Chaos Apothecaries
Chaos being what it is, their apothecaries range from nonexistent to derp to mad scientist. Between the lack of institutionalization and the high priority placed on Rip and Tear, Chaos apothecaries are rare to nonexistent within a single warband. That said, that's not to say they don't exist, but Chaos apothecaries are too rare to be represented on the tabletop, often being relegated to working on new recruits and playing fluffer to the Chaos Lord instead of the battlefield. Some warbands abandon the apothecary idea completely, using their Dark Mechanicum Biologis allies to harvest and process gene-seed from the dead marines (often including the corpses of loyalists and members of rival bands, if the Lord of the band isn't picky), or even inventing new, more… creative ways to breed new marines.
Few Chaos apothecaries really make a name for themselves. There's that Fabulous Bill guy who has fucked things up for the Imperium with his rampant megalomania and successful genetic experiments (which, by the way, has a huge untapped potential in the fluff). There are also mention of a Nurglite, quasizombified apothecary or two floating around somewhere, and the Red Corsairs having right and proper apothecarion, which have something to do with Huron Blackheart being the only sensible man in charge on the Chaos side. Also, there is Talos of the Night Lords, who is an ex-apothecarius, as mentioned in the book: Death in the void. Template:Marines-Codex