How can we measure risk? One way is to estimate the amount by which the returns vary, or move around. This is sometimes represented by a concept known as "volatility" or "standard deviation". This tells how much the returns of an investment are likely to vary from its "average" return over a given period of time. Low volatility means the returns are very stable and bunched tightly together. Higher volatility means returns fan out more from the average return--you see high or low outcomes more frequently.
Over the last 30 years, the annual volatility of the returns provided by, say, Australian bank bills has been less than 2%. That is, ultra stable. A portfolio of fixed-rate, 10 year Australian government bonds has had more volatility at about 8-9% per annum. This is because with a fixed-rate, as opposed to variable-rate, bond you take a lot of market interest rate risk. The second chart below shows the volatility of the daily returns generated by the top 200 companies listed on the ASX in 2012.
