How TDS is calculated ?

How TDS is computed in case of a salaried employee per month. ?

Suppose a salaried emplyee is having Rs. 25000 as salary and TDS is Rs. XXXX which is deducted per month.

And at year end his salary is Rs 3 Lacs and tax is RsXXX

My question is

  if there is difference in tax calculation at year end on whole salary income and tax thereon and Aggregate of tax deducted wholed year month by month.