People who avoid their legal duty to voluntarily file income tax returns every year, or do not pay the right amount owed to the federal government each year are committing tax fraud. Those who falsify the numbers on their tax returns are also misrepresenting their situation and are committing tax fraud.
There are a number of ways to commit tax fraud, and they include keeping two sets of books, deliberately changing their income to show they make less than they actually do, claiming personal expenses as business expenses, claiming false deductions, entering false amounts in records and bookkeeping files, over reporting how much deductions were, and hiding or actively transferring income or assets.
There are other ways to commit tax fraud, but generally speaking, if the IRS gets wind of someone deliberately evading paying their taxes, they put their full weight behind any investigation to get to the bottom of the issue. In most instances, dealing with the IRS as a private citizen or a business is difficult and non-productive. They have far more legal resources