Declaring a variable means describing its type to the compiler but not allocating any space for it. Defining a variable means declaring it and also allocating space to hold the variable. You can also initialize a variable at the time it is defined. Here is a declaration of a variable and a structure, and two variable definitions, one with initialization:
extern int decl1; /* this is a declaration */
struct decl2
{
int member;
}; /* this just declares the type--no variable mentioned */
int def1 = 8; /* this is a definition */
int def2; /* this is a definition */
To put it another way, a declaration says to the compiler, "Somewhere in my program will be a variable with this name, and this is what type it is." A definition says, "Right here is this variable with this name and this type."
A variable can be declared many times, but it must be defined exactly once. For this reason, definitions do not belong in header files, where they might get #included into more than one place in your program.

You can't declare a static variable without defining it as well (this is because the storage class modifiersstatic and extern are mutually exclusive). A static variable can be defined in a header file, but this would cause each source file that included the header file to have its own private copy of the variable, which is probably not what was intended.

The benefit of using the const keyword is that the compiler might be able to make optimizations based on the knowledge that the value of the variable will not change. In addition, the compiler will try to ensure that the values won't be changed inadvertently. Of course, the same benefits apply to #defined constants. The reason to use const rather than #define to define a constant is that a const variable can be of any type (such as a struct, which can't be represented by a #defined constant). Also, because a const variable is a real variable, it has an address that can be used, if needed, and it resides in only one place in memory.