Biorhythms provide an insight into your intellectual, emotional and physical well being. Biorhythms for these different attributes follow cyclic patterns that commence at birth.
Days on which one of the cycles crosses the zero axis are "critical" days; on these days you should be particularly careful of relying on the affected attribute.
Intellectual
The intellectual cycle is a 33 day cycle and affects your alertness, analytical ability, logic, memory and communication
Emotional
The emotional cycle is a 28 day cycle and affects your mood, creativity, sensitivity, awareness and perception
Physical
The physical cycle is a 23 day cycle and affects your strength, co-ordination, and general physical well-being.
For further information, see the wikipedia entry for biorhythms.