What is Sulfonate esters?

A sulfonate ester $\mathrm{R{-}O{-}SO_2{-}R'}$ is formed from an alcohol and a sulfonyl chloride $\mathrm{R'SO_2Cl}$ in the presence of a base (pyridine). The tosylate, $\ce{R-OTs}$, comes from 4-methylbenzenesulfonyl chloride (tosyl chloride); the mesylate, $\ce{R-OMs}$, from methanesulfonyl chloride. The sulfonate anion $\mathrm{R'SO_3^-}$, the conjugate base of a strong acid and stabilised by resonance over three oxygens, is an excellent leaving group.