client：意为客户，这个只表示银行，广告或律师之类的客户。
customer：意为顾客，可以“买”，但不“用”。customer behavior：a broad term that covers individual consumers who buy goods and services for their own use and organizational buyers who purchase business products. 例如，客服部都是customer service dept. ，消费满意度调查（多见于服务行业）叫customer satisfaction。
consumer：意为消费者，这个消费者意思是“用了买来的商品”，着重“用”这层意思”。consumer behavior：the process through which the ultimate buyer makes purchase decisions. consumer 就指买普通商品。
（来源：知乎）