Stars generate energy in their very centers by nuclear reactions. The main nuclear reaction in the Sun is that two Hydrogen atoms slam together at such energy so that the protons fuse and a new element called Helium forms. When two atoms of Hydrogen come together to form one Helium , the mass of the two hydrogens is a little bit larger than the mass of the Helium. That difference is converted into energy by the formula:

energy= mass difference X speed of light squared (E=mc²).

Planets on the other hand DO NOT generate heat by nuclear fusion. For the most part planets are simply cooling off from their initial state. 4.5 billion years ago the earth was a big globe of molten stuff!, and it has cooled off in the last 4.5 billion years.

A star gets power from nuclear reactions occurring at its core.Planets don't have nuclear reactions powering them. That is the primary difference. They also form in different ways, with stars being formed of mostly hydrogen and light elements, while planets are formed of heavier elements. Many of the heavier elements in planets originally were formed by the nuclear reactions at star cores. So literally you are made of dead star parts. It's pretty cool.

Stars are massive enough that the hydrogen that makes them up is under enough pressure in the center that it can fuse to become helium. This provides the energy that makes the star shine. Planets don't have enough mass to create such stupendous pressures and temperatures that are needed for nuclear fusion.

In general, stars are much more massive than planets and at some point in their "lives" are capable of undergoing nuclear fusion (which is how stars generate their energy). Planets, on the other hand, do not undergo nuclear fusion, and usually orbit stars.

A couple things make planets different than stars. First is the way they move in the sky. Ancient Greek astronomers referred to planets as asteres planetai, or wandering stars, because their position in the sky moves over time. In 2006, the International Astronomical Union, a collection of astronomers whose job it is to name and classify stars and planets, declared that a planet in our solar system must orbit the sun, must be round in shape, and must have a much larger mass than anything else in its orbit. The makeup of planets is also different than stars. Planets form from an already existing star when matter (solid or gas) from the star begins to condense around ice or rock cores to form a rock/ice planet (for example, the earth) or gas around a central core (for example, Jupiter).Stars are also classified as having high enough temperatures to perform nuclear reactions in its core that burn hydrogen. As it turns out, to have these high temperatures, a star needs a mass of at least 75 times the size of Jupiter. Therefore, a star is also much larger than any of the planets in our solar system.