Here are the simple terms of the zones you learn and each persons heart rate differs obvioucly with each zone:

Base: Long duration and recovery exercise. Primarily stressing (training) the aerobic system. Training in this zone
time and maximizes development of the functional capacity of the central circulation (i.e. your bodies ability to transport remove waste). It also enhances aerobic capacity of the specific muscles and the systems within those muscles.
T1: Where many people train. Not predominently aerobic or anerobic. In this zone the aerobic or threshold system are not being trained specifically. While the average heart rate may fall into this zone it is not suggested to train in this zone exclusively.
T2: High quality, pace and interval type training. Stressing both anaerobic and aerobic systems. Training in this zone requires a greater amount of recovery between workouts breakdown of both the anaerobic and aerobic systems is high. This type of training is threshold specific training and may compromise the aerobic system or cause overtraining if used too much.
SS: Speed work. Stressing the anaerobic system. Training in this zone requires the greatest amount of recovery and overtraining is likely if used too much. Above AT trains an individual for short duration high intensity bouts shifting metabolism to anaerobic and possibly compromising the aerobic system.
SIGNS OF OVERTRAINING: Unexplained poor performance and high fatigue ratings, Need for prolonged recovery from typical training
sessions, Disturbed mood states, Persistent feelings of muscle and/or joint soreness and stiffness, Overuse injuries, Altered perception of effort at relative heart rates.