declaration : int a; 
declaration means here a is declared as integer variable 
initialization : int a = 5;
initialization is whatever you have declared give that 
variable some value
definition : int a;
definition is whatever variable you have declared give it's 
datatype as here a is declared as integer variable

declaration means you declare variable.
ex: int a;
defination means this integer type a variable take 2 byte 
memory space.
initialization means we initialize the value of integer a 
variable type. 
ex: int a=2;
in declaration we declare variable and variable type , in 
initialization we define the value and in defination we 
define the type of variable.

A good example to illustrate this Question ges like this:

   Whenever we apply for a job the company declares that we 
have been selected. Similarly we declare a variable like 
 
    int emp;
         (here emp behaves as an employee).

   Then he will be assigned with a initial salary which 
will be incremented in course of time period or on 
experience basis.
        Similarly the variable int is assigned a initial 
value.
            i.e int emp=20; 
             (value can be changed in between program)

      The employee is now identified by his post, which can 
define his status in the company.
          Same way the variable is also defined in the 
program.
             ie. int emp;

As i know.....
Definition is one which allocates memory.
Ex: int x;

Declaration never occupy any memory space.
Ex:Function prototype declarations such as....
    int func(int,int);

Initialization: If any variable assigned with some value at
time definition itself is called initialization.
Ex:int x=0;