allotrope is a different structural form of the same element
so for example with Carbon atoms, they can form balls called Bucky balls. this is one form. carbon can also be arranged in layers, this is another structural form known as graphite. both are made of pure carbon atoms, but thier structure means they have different properties. carbon soot is another form
there is also a few allotropic forms of phosphorus. there is red and white and some other colours
isotopes are simply two atoms that have the same atomic number (which means they have the same number of protons and therefore are the same element) but which have a different number of neutrons (therefore thier atomic weight is different.)
for example there is Carbon-12 which has 6 protons and 6 neutrons and there is the isotope carbon-13 which has 6 protons and 7 neutrons. there is also carbon-14 which is believe is radioactive.