Hello Shanmukha,
The calculated attribute doesn't get stored in the database, instead every time you retrieve an object it gets calculated.
There are valid use case scenarios for calculated attributes, but generally I suggest you avoid calculated attributes as they can bog down your app easily when working with large sets of data.
Hope this helps.
A stored attribute is an attribute which value is stored in the database.
A calculated attribute (or Calculated Value) is an attribute which value is calculated EVERYTIME the object it is in, is used.
Have a look at the academy (use search in the top right), there are a few sections about the difference between calculated and stored attributes.