They are fixed-income and extremely low risk - exactly what you want to base a stable, constantly in use retirement fund (for example) on. Short of an asteroid taking out half the country, it isn't possible for them to lose even a small fraction of their principle, even in the short term*.
The stock market is similar, though not as low risk (or limited return). Once companies are big and stable enough to be listed on an index, the odds of losing all of your principle are extrordinarily low and your investments should be structured in such a way that absorbing a short term 40% loss shouldn't be that big of a deal (ie, if you are two years
into retirement, you shouldn't have much money in the stock market anymore)
Derivatives, on the other hand, are much more like a plain/ordinary casino gamble, easily possible to lose everything you put into it. [edit] Actually, it is more like a casino where they give you credit. It is actually possible to lose
more than you put into it, via credit.
*Let me quantify that: the median home price in the US is down something like 15% from its peak (having trouble finding the exact, but here's a link that says 12.4% in 2008, where most of the loss was:
http://www.inman.com/news/2009/02/12/median-us-home-price-falls-124 ). That means that a mortage-backed security has a hard asset backing 15% lower than when it was bought. And in order for that theoretical 15% loss to become real, people have to default on their mortgages.