Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means that isotopes of the same element have the same atomic number but different mass numbers. The different number of neutrons in isotopes results in variations in atomic mass.
One common example of isotopes is carbon. Carbon-12, Carbon-13, and Carbon-14 are three isotopes of carbon. They all have 6 protons, but the number of neutrons differs, giving each isotope a different mass number.