"Dozens of temporary tax provisions are scheduled to expire at the end of 2013 under current law. Most of the provisions set to expire in 2013 have been part of past temporary tax extension legislation. Most recently, many temporary tax provisions were extended as part of the American Taxpayer Relief Act (ATRA; P.L. 112-240). Collectively, temporary tax provisions that are regularly extended by Congress--often for one to two years--rather than being allowed to expire as scheduled are often referred to as 'tax extenders.' […] There are several reasons why Congress may choose to enact tax provisions on a temporary basis. Enacting provisions on a temporary basis provides legislators with an opportunity to evaluate the effectiveness of tax policies prior to expiration or extension. Temporary tax provisions may also be used to provide temporary economic stimulus or disaster relief. Congress may also choose to enact tax provisions on a temporary rather than permanent basis due to budgetary considerations, as the foregone revenue from a temporary provision will generally be less than if it was permanent. The provisions that are scheduled to expire in 2013 are diverse in purpose, including provisions for individuals, businesses, the charitable sector, energy, community assistance, and disaster relief. Among the individual provisions scheduled to expire are deductions for teachers' out-of-pocket expenses, state and local sales taxes, qualified tuition and related expenses, and mortgage insurance premiums."
CRS Report for Congress, R43124