Schedule I is Completely illegal to produce, sell, distribute, and possess.
Schedule II is a substance that has medical value, but has high abuse potential. Schedule II substances are closely watched and are a hassle to deal with. Adderall, Oxycodone, methamphetamine, and cocaine are in this category.
Schedule III is a substance that has medical value but has a lower abuse potential.
Schedule IV is the same with even lower abuse potential.
Schedule V is a substance that is medically useful and has very low abuse potential. These are usually available behind the counter at a pharmacy without a prescription, but many pharmacies don't sell these because they don't want to deal with the liability.
These are all for the US.