What is the difference between these two Canon lenses?
One is the older Canon EF-S 18-135mm f/3.5-5.6 IS and one is the newer Canon EF-S 18-135mm f/3.5-5.6 IS STM.
The newer STM version is an all around better lens. It has slightly improved optics for slightly better image quality. It also has STM (stepper motor) Auto Focus which is faster and virtually silent.
You can tell the difference in photos because the STM version clearly says "STM" on the front and it has "Canon" closer to the front of the lens barrel.