Axial tilt is an astronomical term regarding the inclination angle of a planet's rotation axis in relation to its orbital plane. A planet whose rotation axis were completely perpendicular to the orbital plane would have an axial tilt of 0°. In our solar sytem, the orbital plane that contains the earth is known as the plane of the ecliptic.
Earth has an axial tilt of 23.5°. The Earth is tilted in the same direction throughout a year; however, as the Earth orbits the Sun, the hemisphere tilted away from the Sun will gradually come to be tilted towards the Sun, whereas the hemisphere tilted towards the Sun will come to be tilted away from the Sun.
Axial tilt is partially responsible for the seasons. During part of the year one hemisphere (or the other) is tilted toward the Sun, resulting in longer days and shorter nights; during the rest of the year the hemisphere is tilted away from the sun. Not only does axial tilt cause the hours of sunlight to vary but it can also result in sunlight striking the ground at a more (summer) or less (winter) perpendicular angle.
Although the angle of the earth's tilt does not change perceptibly, the direction of the tilt gradually undergoes precession, moving in a slow circle over a period of about 25,800 years. However, other factors may change the axial tilt of the earth (and of other planets).