Phobos is one of the least reflective objects in the solar system, having characteristics similar to C or D-type asteroids. It is composed of rocks which are rich in carbonaceous material, much like C-type asteroids and carbonaceous chondrite meteorites.
Phobos orbits Mars at a distance of 9,377 kms, and is only 22 kms across. Its density is low, and is believed to be porous in nature. There might be huge deposits of ice underneath its surface covering the dust. It takes 7 hours and 39 minutes to complete one full orbit, because it orbits faster than Mars, as Mars takes a day to complete an orbit.
If viewed from the Earth, it would cross the sky in 4 hours, and appear to be ⅓ the size of the Earth's moon. Since the gravity of Mars is slowing down its orbit, it is estimated that in the coming 10-15 million years, Phobos will slowly descend until it passes within the Roche limit of Mars' gravity.
If this happens, the moon will be torn apart into a string of debris. A few million years after that, these debris will rain down on Mars creating a line of craters. There are already many lines of craters on Mars, indicating that it is most likely that this process has happened before.
Deimos is smaller than Phobos and orbits further away from Mars. It is named after a Greek mythological figure which represents 'dread'. Deimos has most of its characteristics and shape similar to that of an asteroid and was believed to be captured by Mars millions of years ago.
Like Phobos, it has spectra, albedos, composition, and densities similar to C or D-type asteroids. It is also non-spherical in shape, and measures 12 kms across, orbiting Mars at a distance of 23,460 kms.
Deimos is cratered, but due to the partial filling of the craters with regolith, it has a more smoother surface than Phobos. It takes about 30 days or 1.2 Martian days to complete its orbit. From Mars, it appears like a star to an unaided eye.
NASA's Mars Exploration rover has observed these moons passing directly in front of the Sun. When Phobos passes, it is known as an eclipse on Earth, but technically it is just a pass through, as the moon doesn't block the disk of the Sun. On the other hand, Deimos is too small to cause an eclipse, and looks like a black dot moving across the face of the Sun.