What is Phosphonium ylides?

A phosphonium salt $\mathrm{R_3P^+{-}CH_2R'\ X^-}$ is made by bimolecular substitution of a halogenoalkane by a phosphine, usually triphenylphosphine $\ce{Ph3P}$. Removing a hydrogen from the carbon bound to phosphorus gives a phosphonium ylide, a neutral molecule with adjacent opposite charges, $\mathrm{Ph_3P^+{-}CH^-{-}R'}$, also written $\mathrm{Ph_3P{=}CHR'}$. A stabilised ylide carries on that carbon an electron-withdrawing group (an ester, a ketone, a nitrile) that delocalises the negative charge.