Every year, there are two equinoxes, in March and September, when the sun shines directly on the equator and the lengths of day and night are about equal. The days are exactly 12 hours long during an equinox.
Equinoxes occur because the earth is spinning but it's axis of rotation is not perpendicular to its surface. So at the equinoxes, the plane of the earth's rotation is right down along the equator, making the distance from the north pole to the south pole zero at these times.
At other times of the year, one side of the globe gets more sunlight than another - summer on the northern hemisphere, winter on the southern. These differences between the seasons cause changes in the amount of land and water that get heated by the sun. They also cause changes in the direction of the earth's rotation, which means that some parts of the planet get dark at night while others remain illuminated.
The times of the year when this happens depend on where you are on Earth. If you're near the North Pole, you'll experience polar nights in winter and polar days in summer. In contrast, people who live near the Equator always have a year-round daylight hour.
At the two equinoxes, the Sun is directly above at "high-noon" on the equator twice a year. The spring (or vernal) equinox occurs on March 20, while the autumn (or autumnal) equinox occurs on September 22. Except near the equator, the equinoxes are the only days that have equal amounts of daylight and darkness. At the equinoxes, the Sun is directly over the center of Earth during the day and night.
At other times of the year, the angle between the Sun and Earth is not exactly 90 degrees. The distance between them changes all the time because we are both rotating around our axis while orbiting around the Sun. At points in space where this orbit crosses the plane of Earth's orbit, there are nights and days of varying lengths. In summer, when the Earth is closest to the Sun, there are days and nights of 12 hours each. In winter, when it is farthest from the Sun, there are also days and nights of 12 hours each, but these days occur when the Earth is south of the Sun.
At the location of any given point on Earth's surface, there is an angle called the zenith distance. This is the maximum height above the surface that can be seen directly overhead. If you were to climb to the top of a high building or mountain, then you could look up and see the sky directly overhead without being obscured by any clouds or other objects. This is called "having view of the sky."
The equinox occurs when the sun shines vertically over the equator. Twice a year, on the March equinox (usually around March 21) and the September equinox (typically around September 22). The days are of equal length at this time.
During the other months, the sun is not directly overhead at the equator; instead, it is below the horizon.
The reason for this variation is simple: location. If you were to stand on the equator, you would see the sky change every hour, because the entire celestial sphere rotates with respect to you. But since the Earth is not exactly aligned with the center of the universe, there are other objects besides just the moon and stars visible during parts of each day. At certain times of the year, these objects rise above the horizon.
For example, during the equinoxes the sun is directly over the equator, so it is not obscured by any other object. But because the earth is tilted on its axis, at other times of the year the path that the sun takes across the sky is obstructed by the planet itself. For example, in January the sun appears to set somewhere between Alaska and Hawaii - but if you walk east or west along the 49th parallel, you will eventually reach land where the sun has set!