If you use the professional services of someone such as a lawyer or an accountant, you are one of their clients. When you buy goods from a particular shop or company, you are one of its customers. Doctors and hospitals have patients, while hotels have guests. People who travel on public transport are referred to as passengers.
接受律师、会计师等的专业服务的人就是他们的client(委托人);从某个商店或公司购买商品的人就是他们的customer(顾客)。医生、医院接待patients(病人)，旅馆接待guests(客人)。乘公共交通工具的人叫做passengers(乘客)。