Robbery is an aggravated form of theft, the theft being aggravated by the threat of force and fear involved immediately before the theft or during the course of the theft. It depends on the sequence of events as to what is a robbery, see below for example.
Example: B is walking along the street when C approaches and pulls out a knife and says "Give me your wallet now or I will stab you". B is frightened and hands over his wallet, C takes the wallet and runs off. This is a robbery.
On the contrary, B is walking along the street counting some money in his hand when C comes along and takes the money from his hand and then punches him. This is not a robbery, but a theft and an assault as the element of fear/force happens after the theft.
Either way you should contact the police straight away to report the incident.