Seasons

Some people think that the seasons are caused by Earth being close to the Sun in summer and far away from the Sun in winter.

Actually, the seasons are caused by the TILT of Earth's AXIS, which is an imaginary line, that extends vertically from the North Pole, through this planet, and comes out to the South pole. It is tilted at 23.5° from it's perpendicular to it's orbital plane. When the Earth is on a particular side of the Sun, one hemisphere receives more direct Solar radiation then the other, that is while we have winter, and celebrate Christmas in the cold, Australians are surfing and celebrate St. Nick's day in the heat. Six months later when the Earth's position is on the opposite side of the sun, then the Northern Hemisphere receives more direct the solar radiation making for longer days and shorter nights and this is also the reason the Sun looms higher in the sky.
The Earth's axis is tilted from perpendicular to the plane of the ecliptic by 23.45°. This tilting is what gives us the four seasons of the year - spring, summer, autumn (fall) and winter. Since the axis is tilted, different parts of the globe are oriented towards the Sun at different times of the year.

Summer is warmer than winter (in each hemisphere) because the Sun's rays hit the Earth at a more direct angle during summer than during winter and also because the days are much longer than the nights during the summer. During the winter, the Sun's rays hit the Earth at an extreme angle, and the days are very short. These effects are due to the tilt of the Earth's axis.
The changing distance of Earth from the Sun has little effect on what we refer to as seasons. It is instead the tilt of the planet's axis compared to the flat plane of its orbit.

The Earth spins about its axis at a fixed angle to the plane of its orbit. Depending on the planet's orbital position, either northern or southern latitudes can receive both longer hours of daylight and more direct radiation (i.e. at an angle to the atmosphere closer to perpendicular).

Twice during the year (spring and fall), the Sun appears to circle more directly over the equator, and all areas receive similar amounts of sunlight. From late December to late March, the Southern Hemisphere experiences summer while the Northern Hemisphere experiences winter. From late June to late September, the position is the other extreme, and it is summer in the Northern Hemisphere, winter in the southern.

(see related question)
Because the Earth tilts on its axis, in the winter (late December through early March) the northern hemisphere tilts away from the sun and receives less sunlight, and is therefore colder than it is at other times of the year, while the southern hemisphere tilts toward the sun and receives more sunlight and is warmer that it is at other times of the year; in the summer, it is the northern hemisphere that tilts toward the sun and the southern hemisphere that tilts away from the sun. That is why there are seasons. With no axial tilt, the Earth would have exactly the same climate all the time. Polar regions would still be colder than equatorial regions, but whatever the climate was, there would be no seasonal changes.
When looking at the Earth and Sun from space, suppose you see the Earth on the left side of the Sun, and that it's also tilted so that the top of the planet is further away from the Sun than the bottom, so the axis looks like this (Earth is on the left):

(\) (Sun)

It is not the distance that is important, the difference in distance is insignificant overall. It is the ANGLE at which the sunlight strikes the portion of the earth. This is also why the Sun appears lower in the sky, and why the day is shorter

Now, it's important that the tilt doesn't change. Half a year later, Earth will travel halfway around the sun so that it now appears to the right from your viewing position. Because the tilt is still to the left,

(Sun) (\)

Now the days are longer because the northern hemisphere so it receives radiation more nearly perpendicular and it's warmer. The southern hemisphere now has winter because the radiation is less perpendicular.

You can see a similar effect when you shine a torch (flashlight) at a wall. If you hold it horizontal so the light strikes at right angles all areas receive the same amount of light. Now tilt the torch and you will see the shape of the beam striking the wall change. This is exactly what happens due to the tilt of the earth.

At any given time, the hemisphere adjacent to the pole tilted toward the sun is the one that experiences spring and summer, whereas the one adjacent to the pole tilted away from the sun is the hemisphere experiencing autumn and winter.
Because without them we wouldn't have spring, winter, and fall. It would be summer all the time day in and day out.
The earth has a big, imaginary bar through the north pole to its center to the south pole, called an axis. This axis, during different seasons, tilts either toward or away from the sun. When the northern half of the axis is tilting towards the sun, the northern portion of the earth is closer to the sun, and is therefore hotter, and since the south is farther, it is cooler. So, when it is hottest in the north, it is coldest in the south. It works in the other direction, too. When the north is away from the sun, it is cold in the north and warm in the south (winter in the north, summer in the south). It is for this same reason that the temperatures of the equator are fairly constant and nonseasonal. Because they are the center of the earth, their distance from the sun is almost always the same.

The arctic and antarctic. Actually, if you're right at the arctic circle, you won't really experience perpetual dark; it'll still be dusky light for an hour or so even a couple of degrees or more north of the arctic circle.

If you are in Antarctica at all, then your June and part of July are going to be pretty dark.
Polar