What are isotopes and how do they differ from each other?

An isotope is a different form of the same element. They differ from each other by the number of neutrons, however they have the same number of protons and electrons. This results in a different atomic mass.

Lets use an example - carbon:

Carbon (C) can form different isotopes, with the most abundant being C-12 and C-13.

C has 6 protons (stated under the element on a periodic table) and an equal number of electrons to balance the charge. C-12 has 6 neutrons and C-13 has 7 neutrons, because adding the number of neutrons and protons determines the atomic mass of the isotope (electrons have negligable mass).