A personal trainer is a fitness specialist who helps clients reach their fitness goals. A personal trainer usually evaluates before and after a workout program. The initial evaluation is what the personal trainer uses to spot weaknesses. The trainer then writes a specific workout for that client to strengthen those weaknesses. The evaluation that is done after the workout program tests the progress that has been made. They are not allowed to give nutrition programs unless they are also a certified nutritionist, but they can give suggestions about healthy eating.
A good trainer can motivate a client to push themselves to the limit and realize their full potential. They also should look as if they know what they are talking about. Most good fitness specialists have many years of experience under their belt, and not just a certification.