In the normal taxpayer relationship with the Internal Revenue Service, the taxpayer is an employee and via withholding, taxes are paid the U.S. Government by the employer. Basically the employer pays the tax after each payroll run on behalf of all the employees and the corresponding mandated matching taxes (Social Security and Medicare). But in the small business world, this is not the normal relationship.
Estimated tax is a a legal requirement for any taxpayer expecting to owe at least $1,000 by the due date of their tax return. Estimated taxes are paid quarterly so the taxpayer maintains adequate funds on behalf of their account.