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Mercury is the smallest of the Terrestrials, barely larger than the Moon. Mars is about 40% larger than Mercury, but still only half of Earth's size. Venus is a bit smaller than Earth, but not by much. Because of Mars diminutive size, gravity on Mars is only about 40% of Earth's gravity: a great way to lose weight.
Because of the odd nature of Mercury's movement around the Sun, one Mercury day is two thirds of its year. Venus rotates so slowly, and in retrograde fashion, that its day is longer than its year!
Mercury and Venus rotate very slowly: one day on Mercury is 58 Earth days, and one day on Venus is 243 Earth days. A Martian day, by comparison, is just a little longer than an Earth day.
Mercury is the only Terrestrial other than Earth to have a significant magnetic field. This is because it probably still has a molten iron core.
Volcanoes continue to ravage the Earth, but there is no convincing evidence of active volcanoes on the other three Terrestrials. Recently acquired evidence clearly shows strong past volcanism on Mercury. Similar evidence exists for past volcanism on Venus and Mars. For what it's worth, the largest known volcano in the Solar System, Olympus Mons, is not on Earth, but on Mars.
Mercury has no atmosphere, but Venus does: its atmospheric pressure is ninety times that of Earth's. Venus' atmosphere is so dense that the surface of Venus cannot be seen optically. Instead, its contours have been mapped by radar. Mars has an atmosphere too, but it is puny compared to Earth's: its atmospheric pressure is only 1% of Earth's atmosphere.
The first four planets of our Solar System are called the Terrestrial Planets. This is because they have rocky surfaces, like our Earth, and the latin word for Earth is terra. In order of distance from the Sun, shortest to longest, the Terrestrials are Mercury, Venus, Earth and Mars.
Mercury and Venus do not fully cross our sky: they are inferior planets. Typically we see them either in the morning or evening, relatively close to the eastern or western horizons respectively. Mars, being a superior planet, does fully cross the sky. Sometimes, when it is in opposition, it is quite close, and so appears relatively large.
Besides Earth, only one other Terrestrial has a moon, and that's Mars. Mars has two small moons, Deimos and Phobos, and they are believed to be captured asteroids. No Terrrestrials have rings around them, although the Earth might have had one or more "rings" in times past.
Another interesting point of comparison amongst the Terrestrials are their axial tilts: Mercury has none at all, Venus is essentially upside down due to its retrograde rotation, and the Earth and Mars have very similar axial tilts (23.5 degrees and 25.2 degrees, respectively.
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