A tax notice is issued by the income tax department when the Assessing Officer finds any default in the Original/Revised Income Tax Return. A tax payer can get notice from Income Tax Department for many reasons under various sections.
Every tax notice does not demand for additional tax. Notices are issued under different sections and every section has some different demand.
Responding to tax notice is necessary in every case otherwise you have to face the consequences as stated afterwards.
If no action taken by you within 30 days of receipt of the notice u/s 245, the outstanding demand as on that date will be considered for adjustment against your refund.
Non-payment of tax demand attracts penalty and prosecution under various provisions of the Income Tax Act, 1961 and interest @ 1% p.m. or part of the month on the amount of default.