A multiuser system is a computer that is able to concurrently and independently execute several applications belonging to two or more users.
多用户操作系统是指计算机能够同时，独立执行属于２个或多个用户的几个应用请求.
"Concurrently" means that applications can be active at the same time and contend for the various resources such as CPU,memory,hard disk and so on.
同时的意思就是：应用程序在同一时刻能被激活并竞争各种资源比如CPU,内存，硬盘等等．
"Independently"means that each application can perform its task with no concern for what the applications of the other users are doing.
独立的意思是：每一个应用能执行它的任务，而无须关心其他用户的应用在做什么．
Switching from one application to another, of course,slows down each of them and affects the response time seen by the users.
当然，＜在用户看来＞，从一个应用切换到另一个应会降低它们的执行效率和影响它们的响应时间．（seen by the users求助?）
Many of the complexities of modern operating system kernels, which we will examine in this book,are present to minimize the delays enforced on each program and to provide the user with responses that are as fast as possible.
大部分复杂的现代操作系统内核,(are present to 目的)是最小化每个程序的延迟时间，并提供给用户尽可能快的响应速度.
