First, you need to understand the difference in discussing types of investments and types of accounts.
Certificate of Deposits (CDs), money market accounts, mutual funds, and stocks are all examples of types of investments.
401(k), IRA, Roth IRA, and taxable accounts are all examples of types of accounts.
In general, those are separate decisions to make. You can invest in any type of investment inside any type of account. So your question really has two different parts:
Tax-advantaged retirement accounts vs. Standard taxable accounts
FDIC-insured CDs vs. at-risk investments (such as stock mutual funds)
Retirement accounts vs. taxable accounts
Retirement accounts are special accounts allowed by the federal government that allow you to delay (or, in some cases, completely avoid) paying taxes on your investment. The trade-off for these accounts is that, in general, you cannot access any of the money that you put into these accounts until you get to retirement age without paying a steep penalty. These accounts exist to encourage citizens to save for their own retirement. Examples of retirement accounts include 401(k) and IRAs.
Standard taxable accounts have no tax advantages, but no restrictions, either. You can put money in and take money out whenever you like. However, anything that your investment earns is taxable each year.
Inside any of these accounts, you can invest in FDIC-insured bank accounts, such as savings accounts or CDs, or you can invest in any number of non-insured investments, including money market accounts, bonds, mutual funds, stocks, precious metals, etc.
FDIC-insured investments vs. at-risk investments
Something you need to understand about investing in general is that your potential returns are directly related to the amount of risk that you take on. Investing in an insured investment, which is guaranteed by the government to never lose its value, will result in the lowest potential investment returns that you can get. Interest-bearing savings accounts are currently paying less than 1% interest. A CD will get you a slightly higher interest rate in exchange for you agreeing not to withdraw your money for a period of time.
However, it takes a long time for your investments to grow with these investments. If you are earning 1%, it takes 72 years for your investment to double.
If you are willing to take some risk, you can earn much more with your investments. Bonds are often considered quite safe; with a bond, you loan money to a government or corporation, and they pay you back with interest. The risk comes from the possibility that the government or corporation won't pay you back, so it is important to choose a bond from an entity that you trust.
Stocks are shares in for-profit companies. Your potential investment gain is unlimited, but it is risky, as stocks can go down in value, and companies can close. However, it is important to note that if you take the largest 500 stocks together (S&P 500), the average value has consistently gone up over the long term. In the last 35 years, this average value has gone up about 11%. At this rate, your investment would double in less than 7 years.
To avoid the risk of picking a losing stock, you can invest in a mutual fund, which is a collection of stocks, bonds, or other investments. The idea is that you can, with one investment, invest in many stocks, essentially earning the average performance of all the stocks. There is still risk, as the market can be down as a whole, but you are insulated from any one stock being bad because you are diversified.
If you are investing for something in the long-term future, such as retirement, stock mutual funds provide a good rate of return at an acceptably-low level of risk, in my opinion.