The division between dry land and water is also an important feature. If the solid surface of Earth were level, all of it would be under water (as in the planet of Robert Forward's science-fiction book "Flight of the Dragonfly"). In fact Earth has continents sticking out above the water, rocky slabs floating on top of deeper layers which slowly shift in a process known as plate tectonics.
Such shifts and related processes create earthquakes, sudden readjustments of the Earth's crust. The shifts rarely exceed a few meters, but they cause elastic waves to propagate throughout the body of the Earth. By studying those waves, we know that Earth consists of concentric layers. Down to about 35 km (values vary) is the lighter rocky crust, below it to 2890 km the denser mantle, in which several layers are distinguished. Below that is the dense, liquid core, believed (from its density) to be mainly iron, and in its middle the solid inner core of radius 1220 km, or maybe more.
Apart from composition two factors dominate the interior. One is the pressure, caused by the weight of overlying layers: because of it, any borehole significantly deeper than 15 km is likely to be crushed. The huge weight also causes Earth (and other planetary bodies and satellites larger than about 500 km) to be spherical: faced by such huge weight, it makes little difference whether the planet consists of solid rock or of liquid.
The other factor is temperature: as volcanoes and deep mines suggest, the deeper one goes, the hotter the Earth is. The heating seems to be confined to the mantle and caused by the small amount of radioactive uranium, thorium and potassium, elements whose decay time is billions of years. If the rate of heating had continued deeper, material may have turned to gas and caused instability and upheaval (as might have happened in the early Earth, when radioactive aluminum 26 provided extra heating). The radioactive energy production is small: but remember, if you stand on 1 square meter of the ground, all the heat produced in the column of the crust and upper mantle below this area can only escape through that area!
The atmosphere of the Earth is a complex system, in constant motion as it helps remove solar heat from the ground and return it to space. Its lower layers are compressed by those above, so its density drops by half about every 5 km in altitude, a number which changes somewhat with local temperature. Its composition is about 78% nitrogen, 21% oxygen and 1% argon, an inert gas. Oxygen is of course essential to creatures such as us: but it would not be present if plant life did not exist on Earth. Oxygen is a reactive substance, and if it were not constantly replenished by plants (which separate it from CO2), it would quickly dwindle and disappear. Nitrogen is also essential to life, to proteins and nucleic acids.
Life has constantly modified Earth--also tying up carbon dioxide in coral and limestone, also because its vegetation greatly reduces erosion. Does life exist elsewhere in the universe--perhaps, intelligent life, too? No one knows for sure. We probably will never be able to rule out life on other galaxies--they are much too distant.
One condition would be finding an earthlike planet at the right distance from its sun. Extra-solar planets have been detected, but because of observational constraints, they are generally much bigger than Earth ("Jupiters") and much closer to their stars. However, even given the proper condition--barring a scenario resembling the one in the Bible--how likely is it for life to emerge spontaneously, from nonliving material? No one knows, because even the simplest life on Earth has enormously complex chemistry. If life arose here (rather than being imported--another possibility), then the earliest stages of its evolution are extinct.
At least one book has suggested that life is a lucky accident (Rare Earth : Why Complex Life Is Uncommon in the Universe by Peter Douglas Ward and Donald Brownlee, Springer Verlag NY 2000). Maybe future study will clarify this question, but until this happens, we better be careful with the way we handle our planet, because it is not impossible we are the only life, at least within communication range.
Two other distinguishing features of Earth are its moon and magnetism. They will not be discussed in details, because such discussions exist in other parts of this web-page collection.
The Moon is distinguished by having a mass of more than 1% of the Earth's; other planetary satellites are much smaller compared to their planet (except for small Pluto). The unaided eye of any human observer on Mars should be able to tell apart the Earth and its Moon. More about the Earth's satellite can be found in three sections, starting here.
A historical review of the Earth's magnetism is given in "The Great Magnet, the Earth" (first of many sections), and an even larger overview of the region of space dominated by that magnetism is in "The Exploration of the Earth's Magnetosphere."