Assuming you aren’t asking about “The Simpsons”, where the Isotopes are Springfield’s less than stellar baseball team, read on.
Every ELEMENT is defined by the number of PROTONS found in its nucleus.
An ISOTOPE is a particular variant of an element, defined by the number of NEUTRONS found in its nucleus.
Most elements are known to have more than one isotope. Common examples are hydrogen and carbon.
Hydrogen is the first element, and has only one proton. The most common isotope of hydrogen has no neutrons whatsoever. Deuterium is the second isotope, and has one neutron. Tritium is the third isotope, and has two neutrons.
Carbon has 6 protons. Its isotopes are commonly referred to by the total number of protons and neutrons in the nucleus. (This method is commonly used for all elements.) The most common form is carbon-12, which has 6 neutrons. (12 – 6 = 6) Carbon-13 has seven neutrons (13 – 6 = 7) and carbon-14 has eight neutrons.
Isotopes are often discussed in terms of “ABUNDANCE”. Isotopic abdundance simply refers to what percent each isotope contributes to a given sample of the element. In some elements one isotope will dominate (as with carbon and hydrogen), in others two may be very nearly equal in abundance (silver is a good example).
Chemically, the different isotopes of an element react in the same manner. Plants can build cellulose and other chemicals using any isotope of carbon for instance. (The small amount of carbon-14 that they use it what is detected in carbon dating tests.) Despite the chemical similarity, isotopes can have distinct properties as well. Carbon-13 will interact with a magnetic field, so that it can be detected using nuclear magnetic resonance tests (MRI). Carbon-14 is radioactive, which is why it can be used for dating organic materials.