The BMW F 800 S falls into the semi sporty segment and the ST is a sporty touring model. Both models have made their mark thickly. They share the same DNA but the difference is really in the seating position.
Both bikes are great for fine cornering but you will still find a different type of rider on an F800S compared to an F800ST. Many accessories are interchangeable but an ST is more likely to be fitted with a complete set of suitcases whereas the S is more likely to have its brake and clutch levers modified or its exhaust silencer changed.