In its article Q&A: Everything You Should Know About Government Shutdowns the Committee for a Responsible Federal Budget describes government shutdowns as follows:

Many federal government agencies and programs rely on annual funding appropriations passed by Congress. Every year, Congress must pass, and the President must sign budget legislation for the next fiscal year, consisting of 12 appropriations bills, one for each Appropriations subcommittee. Congress has not yet enacted any of the 12 bills for FY 2023 that make up the discretionary spending budget and has instead been funding the government through a CR that was enacted at the end of September. In a “shutdown,” federal agencies must discontinue all non-essential discretionary functions until new funding legislation is passed and signed into law. Essential services continue to function, as do mandatory spending programs.

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Since Congress introduced the modern budget process in 1976, there have been 20 “funding gaps,” including the 2018-2019 shutdown and the one in January 2018, when funds were not appropriated for at least one day. (The hours-long lapse in appropriations in February 2018, though sometimes characterized as a shutdown, did not result in federal employee furloughs.) However, before 1980, the government did not shut down but rather continued normal operations through six funding gaps. Since 1981, 10 funding gaps of three days or fewer have occurred, mostly over a weekend when government operations were only minimally affected.

There have now been four “true” shutdowns where operations were affected for more than one business day. The first two happened in the winter of 1995-1996 when President Bill Clinton and the Republican Congress were unable to agree on spending levels and the government shut down twice, for a total of 26 days. The third was in 2013 when a House and Senate standoff over funding for the Affordable Care Act (ACA) resulted in a 16-day shutdown. The fourth shutdown in December 2018 and January 2019, technically only a partial shutdown because five of the 12 appropriations had previously been enacted, centered on a dispute over border wall funding and was the longest-lasting shutdown at 35 days.

The History, Art & Archives of the US House of Representatives provides a list of funding gaps and whether or not they included shutdowns. The US Office of Personnel Management (OPM) maintains information about shutdown furloughs as well as shutdown contingency plans for agencies.