Robbery is the physical taking of property from another individual by the use of force or the implied use of force.
A woman walking down the street when someone comes up from behind, grabs her purse off of her shoulder and runs away - that is a strong-arm robbery, better known as a purse snatch.
If a man is walking down the street and another person comes up to him and says, "give me your wallet," while showing a knife or other weapon, that's an armed robbery, whether the weapon was used or not.
The physical taking of another person's property through force - or implied use of force - is robbery, whether a weapon is used or not. Use or implication of a weapon just helps us categorize the "type" of robbery.