What is the effect of the Earth's tilt on the seasons?

The Earth's axis is tilted about 23.5° relative to its orbital plane. Because of this tilt, as Earth revolves around the Sun, one hemisphere is tilted toward the Sun for part of the year and receives more direct sunlight and longer days - this is summer for that hemisphere. Six months later, that hemisphere is tilted away and receives less direct sunlight and shorter days - this is winter. The variation in sunlight and day length produces the four seasons.