Tdap (tetanus, diphtheria, acellular pertussis vaccination) should be substituted one time for the good ol' Td (tetanus, diptheria, NO pertussis) booster between the ages of 19 and 64, especially for postpartum women, close contacts of infants aged less than 12 months, and all health-care personnel with direct patient contact. All other boosters can be simply Td.
The purpose of this is to boost waning pertussis immunity in older folks who serve as a reservoir for this bug which can be particularly deadly to little tikes.
Source
Adult Immunization Schedule United States, 2009. MMWR Recomm Rep 2009; 57 (53)