Answer
Under Article 6 of the Revised Penal Code, a felony may be committed in three stages. It is consummated when all the elements necessary for its execution and accomplishment are present. It is frustrated when the offender performs all the acts of execution that would produce the felony, but the felony is not produced because of a cause independent of the offender's will — for example, the victim of a stabbing survives because of timely medical treatment. It is attempted when the offender commences the commission of the felony directly by overt acts but does not perform all the acts of execution because of some cause or accident other than his own spontaneous desistance.
The stage matters because it lowers the penalty: under Articles 50 and 51, a principal in a frustrated felony is punished one degree lower than for the consummated felony, and a principal in an attempted felony is punished two degrees lower.