What is an isotope in layman's terms?

A chemical atom has a certain number of protons. This is known as the atomic number. Hydrogen has only one proton, so its atomic number is one. Ordinary hydrogen doesn't have any neutrons, so the normal isotope also has an atomic weight of one. But a few hydrogen atoms have both a proton and a neutron. This is called deuterium, and it has an atomic weight of two (protons and neutrons have weights that are similar, though not exactly the same). There's a third and even rarer isotope, tritium, that has an atomic weight of three: that is, one proton (else it wouldn't be hydrogen) and two neutrons. The isotopes are the varieties of atom that have the same atomic number but a different atomic weight.