An SMB is a company with a small number of employees, maybe with only a part-time individual managing its data center. As compared to an enterprise, it does not have the same budget capabilities, but still has similar business needs and requirements.
The number of employees can really vary, but for example, Gartner defines a company as an SMB if the number of employees is less than 100 (for small) or less than 999 (for medium). And there are also some criteria based on annual revenue, with less than $50 million (for small) or less than $1 billion (for medium).
The size parameters may also differ depending on the country. For example, in Italy, a small company has less than 50 employees and less than €10 million in revenue, whereas a medium company has less than 250 employees and less than €50 million. There is also the mid-market size called Small and Midsize Enterprise (SME). This size organization has one or more full-time employees dedicated to managing its data centers. However, these individuals have generalist IT skills and have to manage two or more IT-related tasks (backups, databases, network, servers, support, and so on) without a specialized role.
There are also Small Office Home Offices (SOHO), which are usually too small for a real virtual environment, or could be more effective using some kind of public cloud solution. Historically, SMB companies have relaxed requirements, due to the limited budget (compared to enterprise), but the digital transformation is also affecting this type of reality, in some cases more than with the enterprises. The main consequence is that now SMBs can have similar (in some cases, the same) requirements to the enterprises (like a high level of availability), but still without the same type of budget and other specific types of constraints.
They can remain more flexible as compared to enterprises regarding timing, the type of constraints, and the ability to talk directly with the C-level of the company. And, of course, the numbers, as the number of SMBs is usually much greater than the number of enterprises.