Seen from far above the North Pole. Earth goes round anticlockwise once a year and spins anticlockwise once a day. Its axis always points the same way in space (towards Polaris), so for half the year the northern half leans towards the Sun and for the other half it leans away.
The same beam of sunlight (one square metre across) lands on the ground at local noon. When the Sun is low the beam is spread over a bigger patch, so each square metre of ground gets less energy. This, together with day length, is what makes summer warm and winter cold.
Earth spins on its axis once a day. The half facing the Sun has daytime, the other half night. The line between them is the terminator.
Earth goes round the Sun in 365.26 days on an ellipse that is very nearly a circle. It is closest (perihelion, about 147.1 million km) in early January and farthest (aphelion, about 152.1 million km) in early July — the opposite of what you would expect if distance caused the seasons. It also moves a little faster near perihelion (about 30.3 km/s) than near aphelion (29.3 km/s), which is why the northern winter half of the year is a few days shorter than the summer half.
Earth's axis is tilted 23.44° from upright relative to its orbit and keeps pointing the same way in space all year. Around 21 June the northern hemisphere leans towards the Sun: the noon Sun is high, days are long and it is summer in the north and winter in the south. Around 21 December it is the other way round. At the equinoxes (around 20 March and 23 September) neither hemisphere leans towards the Sun and day and night are nearly equal everywhere.
Noon altitude, day length and the Sun's declination use the NOAA solar formulas (within about a minute for day length; sunrise and sunset counted at the Sun's upper edge with refraction). Equinoxes, solstices, perihelion, aphelion and the Sun–Earth distance come from the astronomy-engine library; orbital speed uses the vis-viva relation v² = GM(2/r − 1/a). Seasons here are the astronomical ones, from equinox to solstice; weather services often use whole calendar months instead, and in India the seasons are usually counted differently again (for example the six ṛtu, ऋतु). The exaggerated orbit is only a picture: its eccentricity is about 15 times Earth's so the shape is visible, but the numbers always describe the real Earth.