1. The earth ROTATES on its own axis once in 24 hours, giving us day and night.
2. The earth REVOLVES around the sun once in 364 ¼ days.
3. The earth is TILTED 23½ to the plane of orbit.
 

These three factors could cause the effects of movement to be very simple; however each movement is complicated by the fact that the earth's orbit is elliptical rather than circular.

To understand this, first let's consider an earth that does not ROTATE. One side of the earth would exist in 24 hours of daylight, while the other side would have 24 hours of darkness. As this earth REVOLVED around the sun, one side would be in the light for 6 months, followed by 6 months of darkness.

Now let's consider an earth that rotates and revolves, but is not TILTED. This would mean that every spot on the earth would receive exactly 12 hours of daylight and 12 hours of darkness in each 24 hour period, all year long.

However, the earth's axis is tilted. Minimum tilt is 21½, maximum tilt is 24½, and the earth is presently tilted 23½ off the perpendicular. This fact, modified by revolution around the sun, gives the earth its seasons. In summer, the Northern hemisphere is pointing towards the sun, giving longer day lengths as you move towards the North pole. The North pole receives 24 hours of daylight on June 21 (the summer solstice). Meanwhile, the south pole receives 24 hours of darkness on that day.

In winter, the northern hemisphere, which is still tilted towards the north star Polaris, has it's back to the sun, and therefore receives shorter and shorter day lengths as you move towards the pole. On the winter solstice, (Dec. 21) the North pole is in complete darkness for 24 hours.

Now we'll really complicate things. The revolution of the earth is complicated by the fact that the earth's orbit is elliptical, and not completely round. If it were completely round, the earth would always be 150 million km away from the sun. However, due to the elliptical shape of the orbit, the earth is farthest away (152 million km) from the sun at APHELION, and closest to the sun (147 million km) at PERIHELION. The distance seems small, but we are talking about a difference of 5 million km after all.

Another complicating factor is that right now aphelion occurs during summer in the northern hemisphere, and perihelion occurs during our winter. We are actually 5 million km closer to the sun in the winter, at least for the time being anyway.

So why do we not notice a distinct heating difference between the seasons on each of these particular days? Lag of the seasons occurs because of specific heat capacity differences.

Procession of the equinoxes: Because of change in eccentricity of orbit every 21 - 22 000 years, aphelion and perihelion will switch causing the earth to be farther away from the sun in January instead of in June. Imaging the effect this will have on our already cold winters.

1970s Milankovitch cycles. When eccentricity (orbit) is most elliptical, aphelion occurs in January, and the tilt away from the sun is maximized all at the same time, there will be another ice age.