Fundamentals of Statistics contains material of various lectures and courses of H. Lohninger on statistics, data analysis and here for more.

Exponential Distribution

The exponential distribution is controlled by the paramater λ, which can be interpreted as rate of failure. λ has to be greater than zero.
Graphic Representation
Applications The exponential distribution may be used to model the length of random time intervals, such as the duration of telephone calls, the time between two calls, or the life expectancy of electronic devices (disregaring any ageing effects).
Mean μ = 1/λ
Variance σ2 = 1/λ2