Computation is the CALCULATION made for Any person of his/her all income earned under all 5 heads of Income (i.e. Salary, House property, PGBP, Capital Gains and Other sources). The Total of all such income (after setting off the Losses carried forward from previous years) taken under various heads are called GROSS TOTAL INCOME.
Then All the Deduction under various Sections e.g. 80C, 80D, 80DD, 80G etc are deducted. Then the resulting value is called TOTAL INCOME.
Now tax is calculated as per Current year tax rates and TDS/TCS/Advance taxes etc are Deducted from this tax amount. The coming value is YOUR NET TAX LIABILITY which should be paid under section 140A as Self Assessment Tax or If Excess TDS/TCS/Advance taxes etc have been paid then Resulting value will be your REFUND.
This calculation is called in technical language THE COMPUTATION.