When you rub a balloon on your hair, the balloon ends up loaded with electrons.
Your empty aluminum can is neutrally charged, meaning it has equal numbers of electrons and protons spread pretty evenly around it. When the negatively charged, electron-loaded part of the balloon is held near the can, it repels the electrons in the part of the can closest to the balloon because a negatively charged object repels other negatively charged objects (like charges repel each other). Since there are now fewer electrons in that region of the can (and thus proportionally more protons), you have induced a positive charge in that area of the can.
The negatively charged balloon then attracts the positively charged part of the can, pulling it toward the can, because a positively charged object will attract a negatively charged object. (It's true that opposites attract!)
As it rolls, the electrons in the can closest to the balloon are constantly being forced away, causing the positively charged area of the can closest to the balloon to be attracted to the balloon. As a result, as long as the balloon is held just in front of the can, the can will roll towards it.