Answer
The decisive difference is the use of violence, intimidation, or force. Robbery, under Article 293 of the Revised Penal Code, is the taking of personal property belonging to another, with intent to gain, by means of violence against or intimidation of any person, or by using force upon things. Theft is the taking of another's personal property with intent to gain but WITHOUT violence, intimidation, or force upon things.
So the same taking becomes robbery rather than theft the moment the offender employs violence or intimidation against a person (for example, a hold-up) or breaks into a house or receptacle to reach the property (force upon things). The presence of violence or intimidation also generally makes robbery a graver offense than theft.