The Earth and the rest of the cosmos are in a state of dynamic balance. Incoming solar radiation is balanced by outgoing radiation heading towards 2.7K outer space, which is essentially in every direction except toward the sun. The only thing that keeps us from freezing solid is this flow of energy from the sun.
The Earth's climate is always changing, but it changes on many different timescales. On short timescales, day to night and season to season, the planet's temperature varies depending on the amount of sunlight falling on it. This is because light particles (ions) enter the atmosphere during the day and heat up the planet, while heavy particles (aerosols) scatter light and cool it down at night. These fluctuations happen every day and weekly and monthly and yearly. Longer term, large ice sheets can grow and shrink due to changes in the amount of sunlight reaching them. Finally, over 100,000 years or more, the entire ocean floor rises and falls due to the expansion and contraction of ice within the earth under the force of gravity.
All these factors work together to create a climate that is both cold and hot at different times in history. If you were to go back in time, you would find very different conditions on our planet. For example, if you flew into the southern ocean now, you would see that most of Antarctica is covered in ice.
The planet is considered to be in "dynamic equilibrium." The equilibrium component refers to how well these two processes are in balance. The Earth has a relatively small mass and is subject to many forces, so it functions as a great balancing act between contraction and expansion.
If the contraction process were to dominate, the Earth would collapse under its own weight. If the expansion process were to dominate, the Earth would fly apart at the seams. As it is, both processes work together to keep the Earth in equilibrium.
For example, if the Earth were to expand too much, its density would decrease and it would become a loose collection of objects floating through space. This is called "spontaneous fission." The Earth has not escaped from its orbit because it has not expanded enough to push itself out.
Similarly, if the Earth were to contract too far, its density would increase and it would become a ball of molten rock. This is called "explosive fusion." Again, the Earth has not escaped from its orbit because it has not contracted enough to pull itself back into a single mass.
Because of the heat transfer from the sun to the Earth, the Earth is not in thermal equilibrium with the sun. The average distance between the Earth and the sun is about 93 million miles (150 million km). Because of this distance, the Earth's orbit around the Sun is not exactly circular; it is more of an ellipse. As a result, the Earth experiences seasons: long days in the summer and short days in the winter.
The angle between the Earth's axis of rotation and its orbital plane is called the obliquity of Earth's orbit. This angle can vary over time because of interactions between the Earth's rotational system and its orbital system. It can also vary slightly from place to place on the surface of the Earth because of differences in height between highlands and lowlands. The current value of this angle is 25.8 degrees; it has changed over time due to phenomena such as mass extinction events, the creation of ice caps/unfrozen water areas, and plate tectonics.
At any given moment, only part of the Earth is facing the sun. The rest is in darkness. This is because when you look at the whole solar system from space, everything appears to be rotating around the center of mass of the system.