Isotopes are the same chemical elements with varying numbers of neutrons. Each isotope of a particular element has the same number of protons and electrons. Each isotope will have a different mass which corresponds to the number of neutrons present.
Lets look at the example of carbon isotopes. Carbon-12, carbon-13 and carbon-14 are isotopes of carbon with masses of 12, 13 and 14 g/mol respectively. The "normal" atomic number of carbon is 6. This means that every carbon atom has 6 protons. Therefore, the numbers of neutrons in the isotopes are 6, 7 and 8 respectively.
Isotopes have specific notation to indicate the number of neutrons. The chemical symbol is used with the number of neutrons as a superscript in the left upper corner of the chemical symbol and the atomic number as a subscript on the left lower corner of the chemical symbol.
For obvious reasons, if an element has a stable isotope(s) those isotope(s) are predominately the most abundant found on Earth and the solar system. There are only three types of elements where this is not true, tellurium, indium and rhenium. Those three isotopes are more readily found in there radioactive form.