Mutual funds are by far the most common investment, in recent years they have started to lose ground against ETFs, but that's only because it's a very similar product. Mutual funds are very easy to invest in and because it is a pooled invested they take away lot of the risk for investors.
Easy to invest in
Most common form of investment
Most banks and financial institutions have their own
Moderately high fees
Names of funds may not be indicative of what the fund is made up of
Choices can be limited to the institution you're dealing with