Overload stimulus -> Acute fatigue -> Overreaching-> Overtraining Syndrome
Based on that continuum overtraining come and go quickly or last for several months. Recognizing the symptoms early and taking the appropriate steps to recover will keep an individual for developing overtraining syndrome and/or an overuse injury caused by overtraining. A quick search online will provide you lists of the physiological, psychological, and performance symptoms to be aware of when you are exercising. Below are a few:
Physiological: Extreme fatigue, unwanted weight loss, unexpected decrease in body fat, hormone levels fluctuate, loss of strength
Psychological: Depression, lack of motivation, irritability, lack of enthusiasm
Performance: Inability to finish workouts, slowed recovery, inability to compete at normal level
To recover an individual will need to rest, hydrate properly, and eat high quality foods. Rest can not be stressed enough in regards to recovering. Your body needs a chance to adapt to the overload stimulus and will need nutrients from food and water to make this happen.
Last year I was training for the Tough Mudder event in August and experienced an injury due to overtraining. Knowing that I would be running about 12 miles for the event I increased the distance, frequency, and intensity of my runs. I developed some mild Achilles Tendentious in my left leg, so I took a day or two off and RICEd(rest, ice, compression, elevation) it to reduce the inflammation. As I resumed training I began to experience the same pain and discomfort almost immediately, this time I tried taping it and cross-training but the damage had been done. I needed to take several weeks off in order to heal up in time to train for a few weeks before the event. I had enough time to take up to a week off and focus on flexibility and strengthen other muscle groups.
The intention post is not to discourage you from working hard and challenging yourself, but to recognize the symptoms of overtraining by listening to your body. There is nothing wrong with taking extra time off when you feel extremely tired, stressed out, or sore from your last training session. Stay healthy injury free and strive for continuous improvement!
 Thomas R. Baechle, EdD & Roger W. Earle, MA, Essentials of Strength Training and Conditioning (Campaign: Human Kinetics, 2008)