A dependent is either a qualifying child or a qualifying relative of the taxpayer. Both types of dependents have unique rules, but some requirements are the same for both. As explained in Personal Exemptions lesson, the taxpayer's spouse cannot be claimed as a dependent, but can be claimed as a personal exemption.
A taxpayer can claim one exemption for each qualified dependent, thereby reducing the taxable income.
If a taxpayer can claim another person as a dependent—even if the taxpayer does not actually do so—the dependent cannot take a personal exemption on their tax return.
Persons who can be claimed as a dependent may file a return for taxes withheld but they cannot claim any exemptions.