An investment bank is a financial institution that typically specializes in large, complex transactions, such as underwriting an Initial Public Offering (IPO), mergers and acquisitions, direct investment into start-up firms, or advising large institutional clients on investments/transactions.
In short, investment banks help create the bridge between large enterprises and the investor. In that sense, IPOs are one way to accomplish this, but they also help businesses secure financing in other ways, such as through bond issues or derivative products.
Some examples of top investment banks today include Goldman Sachs, Morgan Stanley, and Credit Suisse.