What is Carboxylic acid derivatives?

A carboxylic acid derivative is a compound $\ce{R-CO-Z}$ in which the $\ce{OH}$ of a carboxylic acid is replaced by another group Z. The acyl group $\mathrm{R{-}CO{-}}$ is common to all of them. In an acyl chloride, Z is Cl; in an acid anhydride, Z is $\mathrm{O{-}CO{-}R'}$, two acyl groups sharing one oxygen; in an ester Z is $\mathrm{OR'}$, in an amide $\mathrm{NR'_2}$.