The Earth is tilted in relation to the sun. Earth’s axial tilt to orbit is 23.44°. On March 20th 2013, this marked the Spring Equinox for the northern hemisphere. The Fall Equinox is in September and vice versa for the southern hemisphere. The Equinox is the point at which the Earth is neither leaning towards or away from the sun. The sunlight that hits the Earth is evenly distributed across the northern hemisphere and the southern hemisphere. From space, the terminator (where night meets day on the Earth’s surface) appears to be perfectly straight from north to south.
After the equinox, the sunlight begins to either become more direct or less direct depending on which hemisphere you live in. The Solstice is the point at which the sun appears at either it’s highest or lowest point in the sky or when the Earth is tilted all the way towards or away from the sun. These are known as the summer and winter solstices.
Interestingly, the orientation of the Earth’s axis actually changes very slowly before making a complete circle every 25,800 years.
The illustration above shows how the tilt of the Earth and it’s location in orbit around the sun, change how light falls on the Earth’s surface. For a really cool view of the Earth undergoing seasonal changes in sunlight, you can view this NASA animation made from images from a geosynchronous satellite above Earth.