Avogadro's number, NA, represents the number of atoms or molecules that are in a mole of a substance. This number is 6.0221415 x 1023. Just like 12 things are in a "dozen", 6.0221415 x 1023 atoms or molecules (see Figure 1) are in a "mole". An NA amount of anything is an extremely large number of it, therefore it is only useful when describing amounts of tiny things like atoms or molecules.
This number is extremely important in chemistry and even has a special day of its celebration, known as "Mole Day." It begins at 6:02:21 AM on October 23rd (6:02:21 10/23).