The Moon is Earth's only natural satellite. It has been known to humanity since prehistoric times. It is also the only heavenly body where man has set his foot upon (there were 12 pairs of feet to be accurate!). Apart from many unmanned probes (e.g. Luna, Surveyor) between 1969 – 1972 six  Apollo vehicles landed there (they were Apollo 11 to 17 without Apollo 13 being lost on its journey with the crew luckily coming safely back to Earth!). Further Apollo missions have been cancelled mainly due to their tremendous cost and also some political reasons. During the cold war between the US and the Soviet Union the Americans wanted to restore their image ruined by Russians being the first to send an artificial satellite and a human in to space. Since Americans won the race to the Moon, further manned lunar missions gradually declined in importance leading to cancelling the program altogether. Therefore humans haven't landed on the Moon for over... 30 years! And still there is not much heard of any return to the Moon nor building a Moon base or lunar astronomical observatories. 

    The Moon is in a prograde elliptical orbit (→eccentricity e = 0.0554) with a →semimajor axis a = 384,400 km. At pericenter (closest to the planet) The Moon is separated from the Earth a distance of q = 363,104 km. At apocenter (furthest from the planet) this moon is separated from the Earth a distance of Q = 405,696 km – which is very close in cosmic scale! The mass of the Moon ~7.3477×10²² kg is comparable to about 10,000,000 a mountains like Giewont! (and has been used on this site as a reference unit for comparing the masses of other moons in the Solar System!)
    Tidal forces have slowed the Moon's spin to the point of the rotation period being equal to its orbital period. Therefore we can see only its near side. The eccentricity of its orbit and inclination to the Earth's equator (changing between 18.3 and 28.6 deg) are causing →libration, which allows us to see 59% of the Moon's surface! The rest could only photographed by space probes. Substantial changes in the angular size of its disk caused by the shape of its orbit are not usually noticed since they are too slow (they should not be confused with an imaginary change in its apparent size when just above the horizon being a poorly studied phenomenon of human perception).
    The animation at the top of the page shows the Moon's revolution in fast forward exposing its "dance" caused by libration...

    Looking at the Moon with the naked eye one might even spot some detail. What we are able to distinguish are lunar "seas" basically occuring only on the near side (the reason for this has not been fully understood). Those are not really seas of water but of basalt meaning solidified lava. The Moon is not massive enough to sustain any atmosphere and therefore it is unable to support liquid water... The origins of those "seas" was an unsolved mystery for a very long time. They appear to be scars left over by some cosmic catastrophes and filled with molten lava. All in all, according to modern theories the Moon itself is supposed to originate from a cosmic cataclysm. Probably just after Protoearth was born, some 4.4 billion years ago, it was hit with an object the size of Mars (the Pacific might actually be a reminder of that event!). After the collision a tremendous amount of matter were blown in to space and later formed our Moon...
    Currently it is ever more accepted that without the Moon the Earth would not be able to sustain life! It acts like some kind of a stabilizer. Without it our planet would constantly change the inclination of its axis. If it at some point started spinning in a similar manner to Uranus, one day would last 365 our days. The ocean on the day side would evaporate right down to the bottom and on the night side they would get completely frozen! As you can see the lack of a moon would not only mean less picturesque dates but simply... we wouldn't be here!

    Another characteristic feature of the Moon's surface are the craters, mostly of impact origin. There are miriads of them which means that had the Moon not been our bodygurad those cosmic projectiles could strike the Earth (which brings us back to the matter of life being able to survive in such circumstances!).
    As for the craters they are best viewed when close to the line dividing the sunlit and dark hemispheres called the  →terminator. Full Moon brings out only the prominent rays coming out of some craters...
    For many years their origin had been unknown until some photos taken by space probes from small distances showed chains of craterlets with diameters of up to several meters. At the eyepiece those little craters merge in to one solid streak which appears to be brighter than the surrounding terrain since it has a larger area for reflecting sunlight. Those craterlets were created by the matter falling down after the formation of a large crater and that's why the streaks appear to originate there. A larger impactor could tear up more matter from the surface forming a more prominent system of streaks. Older craters were deprived of their rays since they were erased by lunar erosion similar to the Earth's but caused by other factors.