List of investments from lowest to highest risk?

The term "risk" unfortunately is very qualitative. It is generally defined as "The probability that an investment will not earn its expected return".

Bank CDs are generally considered very low risk. This is because if the bank CD promises to pay you 1.5% annually. There is a very high probability that you will actually earn 1.5%. Even if the bank goes under the FDIC will insure your deposit, so it can be said that bank CDs have a low probability of not earning their expected return and are thus very low risk.

Government bonds are similar. If the US government promises to pay you, they will. The US government has never defaulted on a bond. If they needed more money they could always print it. This wouldn't be very good news because when you got your payment back it wouldn't be worth as much as when you put it in, but at least there is a very low probability that you would not receive your expected rate of return. US government bonds do fluctuate in price however between when you buy them and when they mature, so in that regard they carry some risks, should you need to sell them before they mature.

Next on the totem pole then are money markets and short term corporate/municipal bonds. Because the bonds here are short term, they do not fluctuate much in price, and it is less likely that any corporation or municipality will go bankrupt in the short time frame of 1 year. Therefore it is probable that you will get your expected interest rate. However corporations do occasionally go bankrupt, and these investments can lose money, so they are more risky than government bonds or bank deposits, but they do pay higher interest rates.

Long term corporate/municipal bonds would then be considered next risky. These can fluctuate dramatically in price with changing interest rates, corporations can fail over a 10-20 year period due to unforeseeable events, etc etc. However these bonds tend to fluctuate in price much less than stocks, and while the probability of default is still there, investors can expect a fixed interest payment and principle.

Next would be Blue Chip stocks. These are large companies in industries which tend to be very stable. These companies are less likely to fail, and tend to achieve much more consistent growth and cash flows. For example Utility companies and consumer staples like Proctor and Gambe, Kimberly Clark, or Walmart, tend to earn very consistent earnings and pay very consistent dividends every year. Due to this, they are considered to be lower risk and most investors are willing to accept a lower rate of return from them then they are from more risky stocks.

Next would be moderate stocks. These might include large corporations with low debt, and good management however they are in more unpredictable or competitive industries. For example oil companies make huge amounts of money, but occasionally an oil spill can threaten a company (like BP) and investors might not receive the dividends they expect. This can also cause the stock price to fluctuate wildly and it can be difficult to guess over the long haul how fast a company like this will grow and where its stock price will ultimately go.

Next would be high risk stocks. These might be companies in cyclical industries like airlines, hotels, or auto manufacturing. These companies do well in good times, but are pounded in bad times. Since it is very difficult to predict their rates of return, they are considered more risky and thus tend to sell at lower prices (and higher expected rates of return, although these expectations are hard to predict).

Finally would be distressed stocks and distressed debt. These include companies which are on the verge of bankruptcy. Often times it includes penny stocks or very high yield debt. If you do your research you can profit off of the bankruptcy, or if the company does survive, make huge gains on the recovery. However the risk of you losing all of your money is high, and achieving exactly what rate of return you might expect is impossible. These are some of the highest risk investments and are speculative in nature.

Generally investors will try to build a portfolio that is a combination of different risks in order to build a portfolio that they are comfortable with because it is low enough risk (meaning it will generally fluctuate in price less) but yet high enough risk that they can still earn a decent return. They might build something with 10% short bonds and money markets, 25% long term bonds, 50% blue chip stocks, and 15% higher risk stocks (for example).