When alphabets, digits and special symbols are used properly combine to form constants, variables and keywords.
Now let us understand meaning of these words.
A constant is an entity that does not change, whereas, a variable is an entity that may change and keyword is a word that carries special meaning
In almost every C program there is a lot of calculations involved and the results of these calculations are stored in computer’s memory. Computer memory consists of millions of cells like human memory does. To fully understand where data is stored names are given to memory. Since the value stored in memory changes, the names given to these location are called VARIABLE names.
Let us understand this with help of an example:
| 9 | X |
Consider the memory location shown above. Here 9 is stored in memory location and a name X is given to it.
| 5 | X |
Now we have assigned a new value to X and that is 5. This can overwrite the earlier value 9, since a memory location can only hold one value
So we concluded that that location whose registered name is X can hold different values hence it is known as VARIABLE. And if value of X doesn’t change it is called CONSTANTS.