An investment bank is a financial intermediary that performs a variety of services
Investment banks specialize in large and complex financial transactions, such as underwriting, acting as an intermediary between a securities issuer and the investing public, facilitating mergers and other corporate reorganizations, and acting as a broker and/or financial advisor for institutional clients
Most investment banks maintain prime brokerage and asset management departments in conjunction with their investment research businesses
All investment banking activity is classed as either "sell side" or "buy side". The "sell side" involves trading securities for cash or for other securities (e.g. facilitating transactions, market-making), or the promotion of securities (e.g. underwriting, research, etc.). The "buy side" involves the provision of advice to institutions that buy investment services