投资银行属于金融服务公司或公司部门的某些活动，这些活动包括代表个人、公司和政府的以咨询为基础的金融交易。这种银行传统上与公司融资有关，它可能通过承销或作为客户发行债券或股票的代理人来协助筹集金融资本。投资银行也可以协助参与并购的公司，并提供辅助服务，如做市、衍生品和股票证券交易、FICC服务(固定收益工具、货币和商品)或研究(宏观经济、信贷或股票研究)。大多数投资银行都设有机构经纪和资产管理部门，与投资研究业务相结合。作为一个行业，它被分为凸起的支架(上层)，中间市场(中层企业)和精品市场(专业企业)。
与商业银行和零售银行不同，投资银行不吸收存款。从1933年《格拉斯-斯蒂格尔法案》通过到1999年《格雷姆-里奇-比利利法案》废除，美国一直保持着投资银行和商业银行的分离。包括七国集团(G7)在内的其他工业国在历史上并没有保持这种分离。作为2010年《多德-弗兰克华尔街改革和消费者保护法案》(Dodd-Frank Act of 2010)的一部分，沃尔克规则要求将投资银行业务与商业银行业务在一定程度上分离。
所有投资银行活动都被归类为“卖方”或“买方”。“卖方”涉及以现金或其他证券进行证券交易(如促进交易、做市)，或促进证券(如承销、研究等)。“买方”包括向购买投资服务的机构提供建议。私募股权基金，共同基金，人寿保险公司
Investment banking pertains to certain activities of a financial services company or a corporate division that consist in advisory-based financial transactions on behalf of individuals, corporations, and governments. Traditionally associated with corporate finance, such a bank might assist in raising financial capital by underwriting or acting as the client's agent in the issuance of debt or equity securities. An investment bank may also assist companies involved in mergers and acquisitions (M&A) and provide ancillary services such as market making, trading of derivatives and equity securities, FICC services (fixed income instruments, currencies, and commodities) or research (macroeconomic, credit or equity research). Most investment banks maintain prime brokerage and asset management departments in conjunction with their investment research businesses. As an industry, it is broken up into the Bulge Bracket (upper tier), Middle Market (mid-level businesses), and boutique market (specialized businesses).
Unlike commercial banks and retail banks, investment banks do not take deposits. From the passage of Glass–Steagall Act in 1933 until its repeal in 1999 by the Gramm–Leach–Bliley Act, the United States maintained a separation between investment banking and commercial banks. Other industrialized countries, including G7 countries, have historically not maintained such a separation. As part of the Dodd–Frank Wall Street Reform and Consumer Protection Act of 2010 (Dodd–Frank Act of 2010), the Volcker Rule asserts some institutional separation of investment banking services from commercial banking.
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. Private equity funds, mutual funds, life insurance companies, unit trusts, and hedge funds are the most common types of buy-side entities.