As the Earth travels around the sun, the seasons change. Because our world is tilted on its axis, the sunlight falls on it at different angles during different seasons. The greater the angle of the sun upon, the less intense the sun's rays are when they reach the surface. The less intense the rays are, the colder it is in that part of the world. Between December and February the northern half of the planet angles away from the sun, so it gets cold.
But did you know that when it is winter in Michigan, it is summer in Australia? At the same time the northern half of the world leans away from the sun making it cold, the southern half leans toward the sun, making it warm. In other words, the seasons are opposite for the northern and southern hemispheres.
Another thing that affects temperature is the curvature of the Earth. The farther north or south of the equator we go, the more the Earth is curved at an angle away from the sun. This makes it colder at any time of the year. The north and south poles are curved so much away from the sun that they are always cold. Around the equator, on the other hand, the Earth isn't curved away from the sun much at all, so it is always warm.