Fuel efficiency from distance traveled and fuel consumed.
km/L (kilometers per liter) is "how far one liter takes you" — higher is better. L/100km (liters per 100 km) is "how much fuel a fixed distance costs" — lower is better. They're inverses of the same efficiency, just scaled differently.
MPG (US) and MPG (Imperial) are both derived from the km/L figure, converted using the standard factors (1 km/L ≈ 2.35215 MPG US, ≈ 2.82481 MPG Imperial) — the difference between the two MPG units is that a US gallon and an Imperial gallon are different volumes.
No — any distance and any matching fuel amount works, as long as both were measured over the same driving period. A full-tank-to-full-tank measurement tends to be most accurate since it avoids guessing how much fuel is left in the tank.