Like everything we see in the world, isotopes are a type of atom, the smallest unit of matter that retains all the chemical properties of an element. Isotopes are forms of a chemical element with specific properties.
You can see the different chemical elements on the periodic table.
Each element is distinguished by the number of protons, neutrons and electrons that it possesses. The atoms of each chemical element have a defined number of protons and electrons, but – crucially – not neutrons, whose numbers can vary.
Atoms with the same number of protons but different numbers of neutrons are called isotopes. They share almost the same chemical properties, but differ in mass and therefore in physical properties.
There are stable isotopes, which do not emit radiation, and there are unstable isotopes, which do emit radiation. The latter are called radioisotopes.
Learn more about isotopes here.