求翻译：　　操作系统最基本的结构是模块结构和层次结构。模块结构是最一般的结构化设计。单内核就是模块结构,但内核的概念是一种层次概念。层次结构是建立在系统功能模块分类的基础之上,是一种模块集合作为“层”的结构。操作系统按模块间功能调用方式分单内核与微核两类。另外还有两种应用模型:虚拟机与客户机(服务器)模式,它们是建立在操作系统的系统应用扩展模型。是什么意思？

　　Operating system is the most basic structure module structure and hierarchy. Module structure is the most general structural design. Single-kernel is the module structure, but the core concept is a concept of hierarchy. Hierarchy is a classification on the basis of the establishment of the system function module, is a collection of modules as "layer" structure. By mode-single module function calls between the operating system kernel and micronucleus in both categories. There are also two application model: virtual machine and the client (server) mode, they are based on the operating system's system application extension model.