“Overtraining” in the short-term is known as overreaching. Overreaching may be the result of something as simple as pushing yourself just a little too hard during a workout, or it may be a case of not getting enough rest across a week of training. Overreaching is a precursor to a full-blown case of burnout, which is called overtraining syndrome.
The true state of overtraining takes time to occur, but unfortunately by the time it gets serious you will require lots of extra recovery time to feel "normal" again. It's best to look out for the signs of overreaching and nip it in the bud before you begin to suffer from a full-blown case of overtraining.
To prevent overtraining and burnout, be sure to work within your limits. Besides following a training routine that isn't excessive in volume and eating enough calories to support your lifestyle, it’s important that you listen to your body. If you begin to feel lethargic, unmotivated, experience an appetite increase or decrease, feel depressed, have sleeping difficulties, or stop making progress in the gym, you may be overreaching, and therefore are a prime candidate for overtraining.
Other symptoms of impending burnout include headaches, lowered resistance to common illness, slowed healing, pain in the muscles and joints, and an increase in anxiety level. If you begin experiencing these symptoms, “quit” while you’re ahead. Take some time to rest, and then ease back into a reasonable workout schedule. You’ll feel rejuvenated and hopefully will be able to avoid illness, injury, and generally a setback in your health and fitness.
I hope these tips on preventing burnout from exercise were helpful!