'Isotopes' are elements which have the same number of protons making up the atom's centre – called a 'nucleus' – but differing numbers of neutrons. You can imagine protons and neutrons as very, very tiny red and black balls in a cluster. The red ones, the protons, have a positive charge (like a battery) and the black ones, the neutrons, don't have any charge. They are neutral. But they do have mass (weight) which is exactly the same as the proton's mass. The element called uranium (chemical symbol 'U') is used as fuel in nuclear reactors. It has several isotopes, one of which – the important one which is the actual fuel – contains 235 neutrons and protons and one which contains 238 and so is slightly heavier. So one isotope of uranium is U-235 and the other is U-238. This number is called the 'atomic weight' of an element and many elements have different isotopes which have different atomic weights. What doesn't change in the isotopes of an element like uranium is the 'atomic number'. Uranium, atomic number 92, contains 92 protons and is the heaviest of all natural elements. The rest of the nucleus is made of neutrons, but it is the protons which make an element's chemical nature. An element with 90 protons in its nucleus is different from uranium and is called thorium.
Got that?! It's a little complicated.