Isotopes are atoms of the same element. Remember that an atom is made up of three important components: neutrons, protons, and electrons. Isotopes of an element will contain the same number of protons and electrons but will differ in the number of neutrons they contain.  In other words, isotopes have the same atomic number because they are the same element but have a different atomic mass because they contain a different number of neutrons. This is reflected in the name of the isotope. For example, Carbon-12 tells us that there is 6 neutrons because atomic mass= number of nutrons + number of protons. Carbon-14 tells us that there are two extra neutrons (because the number of protons does not change between isotopes), which means this isotope has 8 neutrons.  

Explain how the isotopes of an element differ.

Oh chemistry.  Matter is made of atoms.  Atoms are made of subatomic particles.  I'll stay simple, so those subatomic particles are protons, neutrons, and electrons.  If an atom has a neutral charge, it has the same number of protons and electrons.  The number of protons (electrons if a neutral charge) is called the atomic number.  It is found in the upper left corner of each element's box on the periodic table of elements.  For flourine, the atomic number is 9.  That means the atom has 9 protons (and 9 electrons if it is neutral).  

Not all atoms of the same element have the same number of neutrons.  All atoms of that element will have the same number of protons, but neutrons can vary.  That's what an isotope is.  Same element, different amounts of neutrons.  Let's use flourine again.  It's possible that a flourine atom has 9 protons, 9 electrons, and 9 neutrons (mass number is 18; 9 pro + 9 neu = 18).  It's also possible that a flourine atom has 9 protons, 9 electrons, and 10 neutrons (mass number is 19; 9 pro + 10 neu = 19).  Same element. Different isotopes. 

The periodic table indicates what an element's typical isotope is too.  Usually at the bottom of the element's box is a number with decimals.  That is the atomic mass.  It's an average of mass numbers for that element.  Flourine's atomic mass is 18.998.  That's really really close to 19.  Flourine has 9 protons.  Mass number of 19 - 9 protons = 10 neutrons.    The typical isotope of flourine has 10 neutrons.