A mole is simply a unit of measurement. Units are invented when existing units are inadequate. Chemical reactions often take place at levels where using grams wouldn't make sense, yet using absolute numbers of atoms/molecules/ions would be confusing, too. Like all units, a mole has to be based on something reproducible. A mole is the quantity of anything that has the same number of particles found in 12.000 grams of carbon-12. That number of particles is Avogadro's Number, which is roughly 6.02x10^23. A mole of carbon atoms is 6.02x10^23 carbon atoms. A mole of chemistry teachers is 6.02x10^23 chemistry teachers. It's a lot easier to write the word 'mole' than to write '6.02x10^23' anytime you want to refer to a large number of things! Basically, that's why this particular unit was invented.