A butt-load of molecules.
the number is like 2300000000000000000000
but you don't need to remember it, it's called the Avogadro Constant and you will be given it in exams.
Some important notes are:
-A Mole of anything in a gaseous state will take up 24 litres of volume
-The chemical mass of every element you see on the periodic table is the number of grams in one mole of that chemical. i.e: Hydrogen has a mass of 1 so 1 Mole of Hydrogen weights 1 gram
Finally: Watch out for when it's used as a measure of concentration because you will often hear teachers say "add 2 moles of Hydrochloric Acid" or whatever, they are actually meaning mol/dm or moles per litre. That is a different figure which basically says how many moles can be found in one litre of that substance