Isotopes are atoms that have same atomic number but different mass numbers.
Isotopes have the same atomic number because the number of protons inside their nuclei remains the same.
They have different mass numbers because they have different numbers of neutrons.
Isobars are atoms that have same mass number (Greek isos equal + Greek barosweight) but different atomic numbers.
Isobars have different atomic numbers because they have different numbers of protons.
They have the same atomic mass because they have just enough neutrons to make the same total of nucleons.