Answer: The axial tilt of the Earth
Explanation: The planet is tilted on its north-south axis by 23 degrees, making one hemisphere (out of northern and southern) closer to the sun than the other for half of the year. The hemisphere that's closer to the earth at any point experiences summer, and the one further away experiences winter.
Answer: The difference between the solstices is that one represents the point at which the hemisphere in question is closest to the sun (summer solstice) and furthest away (winter solstice)