Why is OTs a good leaving group and how does it work? I've been told that $\ce{OTs}$ is a good leaving group for substitution reactions, but I'm not sure how it works or why it is a good leaving group. Can someone please explain? 

$\ce{Ts}$ comes from Toluenesulfonyl chloride, and when the $\ce{Ts-}$ ion replaces and H in an alcohol, it becomes an $\ce{OTs}$ group.
Tosylate groups ($\ce{CH3C6H4SO3-}$) are good leaving groups because their conjugate base forms are quite stabilized by resonance: