What is the physical/special split?

For the first three generations, damage came in two flavours decided by type alone. Every Fire, Water, Grass, Electric, Psychic, Ice, Dragon and Dark move used the attacker's Special Attack, and every Normal, Fighting, Flying, Poison, Ground, Rock, Bug, Ghost and Steel move used Attack, no matter what the move actually described. A punch made of fire was special; a beam of darkness was too.

Pokémon Diamond and Pearl ended that in 2006. From Generation IV onward, every individual move carries its own category, shown by a small icon on its summary screen. Fire Punch became physical, Hyper Beam became special and gained a physical twin in Giga Impact, and Bite, which had already changed category once when it became a Dark move in Generation II, changed back to physical.

The split rescued whole species. Sneasel is the standard example: a fast, sharp clawed attacker whose Ice and Dark types were both special before 2006, so its best stats could never power its own types' moves. After the split it could finally hit with them. Game Freak also wrote new moves to balance the old type chart, adding physical Psychic attacks like Zen Headbutt and Psycho Cut and special Fighting attacks like Focus Blast and Aura Sphere.