One of the major differences is that the rottweiler grows to be around 100 pounds, while the German shepherd is smaller at around 80 pounds. The rottweiler has a slightly shorter lifespan, between 9 and 10 years, compared to the German shepherd, which often lives to age 12.
Other differences include the German shepherd's tendency to be loud while the rottweiler is generally reserved and quiet. Both breeds are intelligent and loyal, but the shepherd tends to be more alert. Rotties are typically more courageous than shepherds, and raised in a family environment, both breeds can be excellent with children.
Both the rottweiler and the German shepherd need a substantial amount of training and socialization to provide physical and mental stimulation and reduce the chance of aggression.
Keep in mind that every dog is different, and these are generalized differences and behaviors. How an owner trains the dog will greatly determine how it behaves.