Phobos and Deimos are two crows, with a special bond to Hino Rei. They are first seen attacking Usagi, pecking at her head. At the commotion, Rei runs outside and throws a slip of paper at Usagi, who promptly falls. (I don't know why they attacked Usagi; Rei says that they usually don't attack people. Maybe they felt the power in her.)
The crows are linked to Rei's power. When Rei first transformed, they were circling above her head; the first time she used her power as Sailormars, they were with her as well. Rei's grandfather says that she met and named them when she first came to the shrine as a little child. She treats them like friends, and laughs and plays with them. But they follow her orders; if she calls them, they come to her aid.
They're more than just Rei's helpers, though. Phobos and Deimos are sometimes able to give Rei messages, despite the fact that they can't talk to her as Luna and Artemis can talk to their girls. Instead, Rei will sometimes see a newspaper clipping or a tarot card nearby, pinned to the floor with a black crow feather.
It would seem that Rei takes comfort from their presence, as well. During the Dead Moon arc, when they visited the Dead Moon Circus, Rei felt uneasy and brought her crows with her. It's as if she draws security from their presence, knowing that kindred spirits are nearby.
It's a good thing that she brought the crows with her to the Dead Moon Circus. That was where she got trapped in the mirror house by Tiger's Eye, and where he stole her confidence. But Phobos and Deimos flew into the mirror house, cawing, and attacked him with raven pinions. Then they transformed into two beautiful sprites, dressed in gray (Phobos) and black (Deimos), with feathers at their wrists and ankles. Their raven hair is done up with a long ponytail in back, and two thick strands in front. Calling Rei "Princess Mars," they restored her heart crystal, the Mars Crystal, to her. Using that power, she transformed, and was able to defeat Tiger's Eye.
It turns out that Phobos and Deimos were two girls from the planet Coronis, chosen to be the partners of Sailormars, and their role was to protect and train Sailormars in her growing power. They went head-to-head with the anima maid Sailor Lead Crow, who recognized them in the Stars arc. They recognized her as a traitor as well, naming her as the one who stole the power of Sailor Coronis, their mother star.
Phobos and Deimos are the two moons of the planet Mars. (For what it's worth, Phobos is a slightly smaller moon than Deimos.) In Greek mythology, Phobos and Deimos are the sons of Ares and Aphrodite (Mars and Venus in Roman mythology). "Phobos" means "fear" in Greek, and "deimos" means "panic." Just as the crows followed Rei into battle in the Sailormoon series, their mythological counterparts accompanied their father into battle.
For a closer look at Phobos and Deimos...