API (Application Programming Interface) is a set of commands, which interfaces the programs with the processors. The most commonly used set of external procedures are those that make up Microsoft Windows itself. The Windows API contains thousands of functions, structures, and constants that you can declare and use in your projects. Those functions are written in the C language, however, so they must be declared before you can use them. The declarations for DLL procedures can become fairly complex. Specifically to C# it is more complex than VB. You can use API viewer tool to get API function declaration but you have to keep in mind the type of parameter which is different in C#.

API(应用编程接口)是程序与处理器接口的命令集。最常用的就是在外部调用微软WINDOWS内部的进程。WINDOWS API包括成千的你可以使用的函数、结构、常量。这些函数是用C语言写的，在使用他们之前，你必须声明。 定义Dll的进程将相当的复杂，甚至比VB还复杂。你可以使用API Viewer工具得到API函数的声明，但是必须注意的是，它的参数类型跟C#的不一样。

Most of the advanced languages support API programming. The Microsoft Foundation Class Library (MFC) framework encapsulates a large portion of the Win32 (API). ODBC API Functions are useful for performing fast operations on database. With API your application can request lower-level services to perform on computer's operating system. As API supports thousands of functionality from simple Message Box to Encryption or Remote computing, developers should know how to implement API in their program.

大部分的高级语言都支持API，微软函数类库(MFC)封装了大部分的Win32 API。ODBC API对提高数据库的操作速度大有好处。使用API，可以请求更底层的系统服务。API从简单的对话框到复杂的加密运算都提供支持。开发者应该知道如何在他们程序中使用API

API has many types depending on OS, processor and functionality.

API有许多类型，（针对不同的操作系统、处理器…………）

OS specific API:
操作系统特有API:

Each operating system has common set of API's and some special e.g. Windows NT supports MS-DOS, Win16, Win32, POSIX (Portable Operating System Interface), OS/2 console API and Windows 95 supports MS-DOS, Win16 and Win32 APIs，

每种操作系统都有一套公用API和专有API。比如：Windows NT 支持MS-DOS, Win16, Win32, POSIX (便携式操作系统接口)，OS/2 console API ；同时Windows 95 supports MS-DOS, Win16 和Win32 API。

Win16 和 Win32 API:
Win16 is an API created for 16-bit processor and relies on 16 bit values. It has platform independent nature e.g. you can tie Win16 programs to MS-DOS feature like TSR programs.

WIN16 是基于16位的处理器，并使用16位的值，它是一个独立的平台。比如：你可以运行TSR 程序在MS-DOS环境下。

Win32 is an API created for 32-bit processor and relies on 32 bit values. It is portable to any operating system, wide range of processors and platform independent nature.

WIN32 是基于32位的处理器，并使用32位的值。他可用于任何操作系统，它的使用范围更广。

Win32 API has 32 prefix after the library name e.g. KERNEL32, USER32 etc?

Win32 API的DLL一般都具有32的后缀，比如：KERNEL32, USER32等。

All APIs are implemented using 3 Libraries.

所有的API都在下面3个DLL中实现的。

Kernel
User
GDI

1. KERNEL
It is the library named KERNEL32.DLL, which supports capabilities that are associated with OS such as

它的库名是：KERNEL32.DLL，它是操作系统管理的API集

Process loading. 加载进程
Context switching.
File I/O. 文件操作
Memory management. 内存管理
e.g. The GlobalMemoryStatus function obtains information about the system's current usage of both physical and virtual memory

比如：GlobalMemoryStatus 函数获得目前系统物理虚拟内存的使用信息。

2. USER
This is the library named "USER32.DLL" in Win32.

在WIN32下，它的库名是 USER32.DLL

This allows managing the entire user interfaces such as

它管理全部的用户界面，比如：

Windows 窗口
Menus 菜单
Dialog Boxes 对话框
Icons etc., 图标等
e.g. The DrawIcon function draws an icon or cursor into the specified device context.

比如：DrawIcon 画一个图标在指定的设备上。

3. GDI (Graphical Device Interface)