When you invest in an investment fund, your money will be managed by a Portfolio Manager and invested in a diversified portfolio, decreasing the risk you bear. At the same time, you are paying a fee for the service, which decreases your returns.
When you invest directly, through a broker-dealer company, you decide on your own where to invest, and the number of different securities you can invest in is limited by the money you have for that purpose. No fees are payable to a management company but the investment risk can be high, if you do not diversify your portfolio adequately. It is also necessary that you possessed some knowledge about the matter and to dedicate some time to analysis of securities and monitoring of your portfolio.
There is a possibility to engage a broker-dealer company licensed to perform portfolio manger activities to invest in securities, instead of you and for your behalf, following their own choice (by their own judgment and assessment) and to create a portfolio for you. On that occasion, you are paying a fee for these services, as with management companies only maybe even higher.