Small businesses are types of corporations, partnerships, or sole proprietorships which have a small number of employees and/or less annual revenue than a regular-sized business or corporation. Businesses are defined as "small" in terms of being able to apply for government support and qualify for preferential tax policy. The qualifications vary depending on the country and industry. Small businesses range from fifteen employees under the Australian Fair Work Act 2009, fifty employees according to the definition used by the European Union, and fewer than five hundred employees to qualify for many U.S. Small Business Administration programs. While small businesses can be classified according to other methods, such as annual revenues, shipments, sales, assets, annual gross, net revenue, net profits, the number of employees is one of the most widely used measures.

Small businesses in many countries include service or retail operations such as convenience stores or tradespeople. Some professionals operate as small businesses, such as lawyers, accountants, or medical doctors (although these professionals can also work for large organizations or companies). Small businesses vary a great deal in terms of size, revenues, and regulatory authorization, both within a country and from country to country. Some small businesses, such as a home accounting business, may only require a business license. On the other hand, other small businesses, such as day cares, retirement homes, and restaurants serving liquor are more heavily regulated and may require inspection and certification from various government authorities.

Researchers and analysts of small or owner-managed businesses generally behave as if nominal organizational forms (e.g., partnership, sole-trader, or corporation), and the consequent legal and accounting boundaries of owner-managed firms are consistently meaningful. However, owner-managers often do not distinguish between their personal and business interests. Lenders also often skirt organizational (corporate) boundaries by seeking personal guarantees or accepting privately held assets as collateral. Because of this behavior, researchers and analysts may wish to be cautious in assessing the organizational types and implied boundaries relating to owner-managed firms. This includes the analysis of traditional accounting disclosures and studies that treat the firm as defined by a formal organizational structure.

The concepts of small business, self-employment, entrepreneurship, and startup overlap but carry important distinctions. These four concepts are often conflated. Their key differences can be summarized as:

Many small businesses are sole proprietor operations consisting only of the owner, but many have additional employees. Some small businesses that offer a product, process or service, do not have growth as their primary objective. In contrast, a business that is created to become a big firm is known as a startup. Startups aim for growth and often offer an innovative product, process, or service. The entrepreneurs of startups typically aim to scale up the company by adding employees, seeking international sales, and so on, a process which is often but not always financed by venture capital and angel investments. Successful entrepreneurs have the ability to lead a business in a positive direction by proper planning, adapting to changing environments, and understand their own strengths and weakness. Spectacular success stories stem from startups that expanded in growth. Examples would be Microsoft, Genentech, and Federal Express which all embody the sense of new venture creation on small businesses.

Self-employment provides work primarily for the founders. Entrepreneurship refers to all new businesses, including self-employment and businesses that never intend to grow big or become registered, but startups refer to new businesses that intend to grow beyond the founders, to have employees, and grow large.

The legal definition of "small business" varies by country and by industry. In addition to the number of employees, methods used to classify small companies include annual sales (turnover), the value of assets and net profit (balance sheet), alone or as a combination of factors.

Small businesses usually do not dominate their field.