overtraining is a totally abused term. it doesn't just mean you over did it in the gym. it could mean lack of sleep, vitamins, nutrients, proteins, or just about anything related to recovery. if you don't allow your body to recover, and you keep on training, you'll get sick, and possibly injured. this is overtraining.
do i believe you can overtrain, sure, everyone has their limits. do i believe that if you sleep the appropriate amount, supplement the appropriate amount, and eat enough that you could overtrain? no, if everything else is in check, i say hit it as fuckin hard as you physically can. but recovery is necessary to grow. and just to elaborate a little, periodization schedules allow for 'active' recovery, meaning you workout while recovering- ie light days. so, any program designed correctly, with diet and lifting schedule, it is almost impossible to overtrain- given everything else is in check.
there are known symptoms of overtraining, as monitored and documented by many sports medical doctors. below is a list of symptoms that indicate overtraining. it's taken from the text- NSCA's Essential's of Personal Training, pg 94, Earle & Baechle.
- Plateau followed by decrease of strength gains
- Sleep disturbances
- Decrease in lean body mass(when not dieting)
- Decreased appetite
- A cold that just won't go away
- Persistant flu-like symptoms
- Loss of interest in the training program
- Mood changes
- Excessive muscle soreness
again, overtraining would be hard to get to if everything else is in check, but it can happen. hope this helps.