An original equipment manufacturer, or OEM, refers to a company that makes a final product for the consumer marketplace. For example, Ford and General Motors are OEM companies that manufacture cars, and Apple is a computer OEM.
Tier One （一级供应商。给设备长厂商供货）
Tier one companies are direct suppliers to OEMs. The term is especially common in the automobile industry and refers to major suppliers of parts to OEMs. For example, Sensata Technologies is a tier one supplier of exhaust gas sensors to automotive OEMs.
Related Reading: Explain the Three-Tier Organizational Structure
Tier Two （二级供应商，给二级供应商供货）
Tier two companies are the key suppliers to tier one suppliers, without supplying a product directly to OEM companies. However, a single company may be a tier one supplier to one company and a tier two supplier to another company, or may be a tier one supplier for one product and a tier two supplier for a different product line. （界限并不是按企业实体分，而是按照某次行为在供应链中的地位来决定）