The Seasons on Earth:

The combined effect of the Earth's annual motion around the Sun and the tilt of its axis cause the seasons.
The length of time the Sun is above the horizon (daylight) changes. The path the Sun follows across the sky varies; the Sun's declination changes.
The noontime altitude of the Sun changes, causing the amount of concentration (area over which rays of sunlight are spread) of the sunlight to vary.
In summer, the Earth is tipped toward the Sun; in winter, away.
When it is summer in the northern hemisphere, it is winter in the southern.
The Sun's declination changes during the year too. Take a look at what happens in Ithaca.