An investment bank is a financial institution that helps corporations, governments, and large investors raise capital, advise on transactions, trade securities, manage assets, and provide research and brokerage services. Crucially, it does not take retail deposits and does not (in its pure form) make loans to consumers. That is a commercial bank's job.
Who an investment bank serves
The clearest way to understand an investment bank is to look at who its clients are. They are not individuals. They are corporations issuing stock or debt, governments selling sovereign bonds, private equity firms buying and selling companies, hedge funds trading and borrowing, sovereign wealth funds and pension plans deploying billions, and ultra-high-net-worth families using private wealth services. The minimum size of a meaningful relationship typically runs in the hundreds of millions or billions of dollars.
Glass-Steagall and the universal bank
Until 1999 in the United States, the Glass-Steagall Act formally separated commercial banking from investment banking. The Gramm-Leach-Bliley Act repealed that separation, allowing the universal bank model that JPMorgan Chase, Bank of America, and Citigroup operate today: one corporate parent with a commercial bank, an investment bank, an asset manager, and consumer-facing operations all under one roof. Goldman Sachs and Morgan Stanley converted to bank holding companies during the 2008 financial crisis to access Federal Reserve liquidity but remain primarily investment-banking-focused. Pure stand-alone investment banks (the old Lehman, Bear Stearns, Merrill Lynch model) have effectively disappeared.
Investment bank ≠ asset manager ≠ hedge fund ≠ PE firm
An investment bank is also distinct from an asset manager (which holds and invests other people's money under a mandate, like BlackRock or Fidelity), a hedge fund (which manages money for sophisticated investors and trades for absolute return), and a private equity firm (which buys whole companies). Investment banks routinely interact with all three but do not do their primary job.
Exercise
For each of the following business activities, identify whether it is performed primarily by an investment bank, a commercial bank, an asset manager, a hedge fund, or a private equity firm — and briefly justify: (1) Underwriting Safaricom's USD 600m eurobond issue. (2) Holding KES 50bn of customer current-account deposits and lending them to SMEs. (3) Running a USD 5bn long-only EM equity mutual fund tracking the MSCI EM Index. (4) Buying a controlling stake in a Kenyan logistics company, replacing the CEO, and selling 5 years later. (5) Advising Bamburi Cement's board on a potential sale to Holcim. (6) Running a USD 2bn long/short equity strategy in African names with 2-and-20 fees.