根据GNU C库手册(http://www.gnu.org/software/libc/manual/html_node/Header-Files.html)

在C语言中，声明仅仅提供函数或变量存在的信息并给出其类型。对于函数声明，还可以提供关于其参数类型的信息。声明的目的是允许编译器正确地处理对声明的变量和函数的引用。另一方面，定义实际上为变量分配存储空间，或者说明函数的功能。

声明说"这个东西存在于某个地方"

int sampleFunc(); // function
extern int car;  // variable

定义说，这个东西存在于这里;为它留下记忆"

int sampleFunc() {} // function
int car; // variable

在定义对象时，初始化是可选的，并表示“这是这个东西的初始值”:

int car = 0; // variable

变量的声明是为了通知编译器以下信息:变量的名称，它持有的值的类型和初始值(如果有的话)。也就是说，声明给出了关于变量属性的详细信息。然而，变量的定义说的是变量存储在哪里。也就是说，变量的内存是在定义变量时分配的。

定义意味着实际编写的函数，声明意味着简单的声明函数 如。

void  myfunction(); //this is simple declaration

and

void myfunction()
{
 some statement;    
}

这是函数myfunction的定义

从wiki.answers.com:

术语声明(在C语言中)意味着你告诉编译器关于类型，大小，如果是函数声明，任何变量的参数的类型和大小，或者程序中用户定义的类型或函数。在声明时，内存中不为任何变量保留空间。然而，编译器知道在这种类型的变量被创建的情况下要保留多少空间。

例如，以下是所有的声明:

extern int a; 
struct _tagExample { int a; int b; }; 
int myFunc (int a, int b);

另一方面，定义意味着除了声明所做的所有事情之外，内存中还保留了空间。你可以说“DEFINITION = DECLARATION + SPACE RESERVATION”，下面是定义的例子:

int a; 
int b = 0; 
int myFunc (int a, int b) { return a + b; } 
struct _tagExample example; 

看到答案。

《K&R》(第二版)中有一些非常明确的定义;这有助于把它们放在一个地方，并作为一个整体来阅读:

"Definition" refers to the place where the variable is created or assigned storage; "declaration" refers to the places where the nature of the variable is stated but no storage is allocated. [p. 33] ... It is important to distinguish between the declaration of an external variable and its definition. A declaration announces the properties of a variable (primarily its type); a definition also causes storage to be set aside. If the lines int sp; double val[MAXVAL] appear outside of any function, they define the external variables sp and val, cause storage to be set aside, and also serve as the declaration for the rest of that source file. On the other hand, the lines extern int sp; extern double val[]; declare for the rest of the source file that sp is an int and that val is a double array (whose size is determined elsewhere), but they do not create the variables or reserve storage for them. There must be only one definition of an external variable among all the files that make up the source program. ... Array sizes must be specified with the definition, but are optional with an extern declaration. [pp. 80-81] ... Declarations specify the interpretation given to each identifier; they do not necessarily reserve storage associated with the identifier. Declarations that reserve storage are called definitions. [p. 210]