Simply put, a tax-deductible expense is one that is recognized by the IRS as being an expense for the purpose of producing income wis subject to tax. Different types of businesses have varying deductions specific to that business, but, on the whole, deductions such as auto expenses, some travel, repairs & maintenance, office expenses, postage, etc. are deductible by all businesses. There are publications on the IRS website such as Publication 535, free for download, that identifies the most common ones.