A per curiam decision is a court opinion issued in the name of the Court rather than specific judges. Most decisions on the merits by the courts take the form of one or more opinions written and signed by individual justices. Often, other judges/justices will join these opinions. Even when these signed opinions are unanimous, they are not per curiam, as the judges'/justices' names still appear.
Per curiam decisions are given that label by the court issuing the opinion, and these opinions tend to be short. The opinions will typically deal with issues which the issuing court views as relatively non-controversial.
Per curiam decisions are not always unanimous and non-controversial. Bush v. Gore, 531 US 98 (2000) is one of the most well-known Supreme Court cases with a majority per curiam opinion that also contains additional opinions. According to this comprehensive 2015 study on Supreme Court decisions, "the Court decided 59.2 percent of the per curiam decisions by a unanimous vote compared with 36 percent of the cases resulting in a signed opinion."
Examples of per curiam decisions
Frequency of per curiam decisions
For an article which advocates against the concept of per curiam opinions, see this Tulane Law article.
And for more on the history of per curiam decisions, see this Journal of Supreme Court History article.