The sun heats the surface of the earth through irradiation. Nuclear reactions in the sun let of a lot of energy in the form of electromagnetic radiation. This radiation is basically just light that travels through space, and some of it reaches earth. The radiation has very high energy and when it hits the surface of the earth, much of it is turned to heat, which increases the temperature of the surface of the earth. Some of the radiation bounces off the surface of the earth and travels back into space. The radiation that bounces of is known as albedo. Certain gases in our atmosphere, especially carbon dioxide (CO₂) and methane (CH₄) trap the reflected radiation in our atmosphere, causing it to heat up. This is why CO₂ and CH₄ are known as greenhouse gases, because they make our atmosphere act sort of like a greenhouse.

Energy from the sun really only heats up the atmosphere and the surface of the earth (including oceans). The solid earth (all of the rock below the surface) is actually cooling down. Heat was generated when the earth formed about 4.6 billion years ago and is still generated as radioactive elements in the earth (particularly potassium, uranium, and thorium) decay. This heat is radiated from the surface of the earth out into space. So, the entire earth is actually cooling down, but the atmosphere and the surface of the earth is warming up due to energy from the sun that gets trapped by greenhouse gasses.