Investments can vary dramatically as there are so many options.
If one wants to invest within the Islamic guidelines, ALL interest-bearing investments or their equivalents are prohibited. This means that all bonds, debentures, and other guaranteed investment vehicles offered by financial institutions and insurance companies, including all balanced mutual funds, DO NOT meet Islamic guidelines. There is no room for interpretation here. What is haram is haram (prohibited).
In terms of investing, Muslims can invest in companies directly if they are conducting business in permitted industries under the correct debt ratios. This extends to the capital markets as well so purchasing shares in corporations that meet strict Islamic criteria as established by venerable International Scholars such as those on the Dow Jones Islamic Market Index are permissible.