- An IRS 90-Day Letter notifies an individual that their taxes were audited and a discrepancy was found, requiring payment unless petitioned within 90 days. - An abusive tax shelter is an investment scheme that claims to reduce taxes without changing income/assets and serves no economic purpose other than lowering taxes owed. People who use abusive tax shelters can face penalties from the IRS including back taxes and interest. - A tax shelter is a legal way to minimize taxable income and tax liability, such as 401(k) plans or investments providing favorable tax treatment.