Earth is the third planet from the Sun and the fifth largest.  It orbits 149,600,000 kilometers from the Sun, has a diameter of 12,756.3 kilometers, and a mass of 5.9736x10 to the 24th kilograms.  Earth is the only planet whose English name does not derive from Greek/Roman mythology.  The name derives from Old
English and Germanic.  There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus - the fertile soil (Greek: Gaia, terra mater - Mother Earth).  Earth, of course, can be studied without the aid of spacecraft.  Nevertheless it was not until the twentieth century that we had maps of the entire planet.  Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes.  The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

                  0 - 40  Crust
               40 - 400  Upper mantle
             400 - 650  Transition region
            650 - 2700  Lower mantle
          2700 - 2890  D'' layer
          2890 - 5150  Outer core
          5150 - 6378  Inner core

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents.  The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid.  The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic (also known as the Moho) discontinuity between the crust and upper mantle.  The core is probably composed mostly of iron (or nickel/iron) though it is possible that some lighter elements may be present, too.  Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun.  The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum.  The upper mantle is mostly olivene and pyroxene (iron/magnesium silicates), calcium and aluminum.  We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily
quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is:

    34.6%  Iron
    29.5%  Oxygen
    15.2%  Silicon
    12.7%  Magnesium
    2.4%  Nickel
    1.9%  Sulfur
    0.05% Titanium

The Earth is the densest major body in the solar system.  Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below.  The theory that describes this is known as plate tectonics.  It is characterized by two major
processes: spreading and subduction.  Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below.  Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle.  There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental plates (i.e. India/Eurasia).  There are (at present) eight major plates:

     North American Plate - North America, western North Atlantic and Greenland
     South American Plate - South America and western South Atlantic
     Antarctic Plate - Antarctica and the "Southern Ocean"
     Eurasian Plate - eastern North Atlantic, Europe and Asia except for India
     African Plate - Africa, eastern South Atlantic and western Indian Ocean
     Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean
     Nazca Plate - eastern Pacific Ocean adjacent to South America
     Pacific Plate - most of the Pacific Ocean (and the southern coast of California)

The Earth's surface is very young.  In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters).  Thus the very early history of the Earth has mostly been erased.  The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare.  The oldest fossils of living organisms are less than 3.9 billion years old.  There is no record of the critical period when life was first getting started.

71 Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water beneath the surface of Europa).  Liquid water is, of course, essential for life as we know it.  The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable.  Liquid water is also reponsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water.  There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants.  Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various "sinks" and back again.  The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect.  The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a comfortable +14 C); without it the oceans would freeze and life as we know it would be impossible.  The presence of free oxygen is quite remarkable from a chemical point of view.  Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements.  The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen.  The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century.  Current research indicates that about 900 million years ago there were 481 18-hour days in a year.