The SME (Small to Medium Enterprises) definitions widely differ country by country. Regardless of the definition SMEs accounted for 90+ percent of all firms and contributed on average of 40+ percent of private nonfarm GDP in 2008. This makes them hugely important for economic growth, innovation, and diversity.
This article published by Susan Ward provides more insight of what SMEs are and their importance https://www.thebalancesmb.com/sme-small-to-medium-enterprise-definition-2947962