The STM stands for Stepping Motor a new autofocus technology claimed by Canon to smoothly and silently focus and achieve continuous AF while recording video when used with the 650D and presumably future.
The Canon EF-S 18-135mm f35-56 IS STM Lens is a significant upgrade from the standard-at-review-time 18-55mm IS kit lens.
Canon 18-135mm lens are amazing for beginners who just wants a standard lens but also wants a tiny telescopic with it.
You can possibly take of anything with these and most likely dont have to keep taking it off your camera body.
Canon EF-S 18135mm f3556 IS STM A new version of the lens the f3556 IS STM was announced on 8 June 2012 alongside the EOS 650DRebel T4iKiss X6i.