PIGS is a term used by economists and journalists to refer in a concise way to the four nations of Southern Europe - Portugal, Italy, Greece, and Spain. These four nations are perceived to share certain cultural and economic traits, and thus commentators have found situations where a concise acronym is useful.
The term rose to special prominence during the ongoing European Sovereign Debt Crisis, wherein these four nations have been among the worst offenders at racking up unsustainable sovereign debt levels. Another major offender in the most recent crisis was Ireland, which is often included in the expanded term PIIGS.
It goes without saying that the ordering of the nations within this acronym was designed to create the most pejorative English term possible, and consequently the nations in question have repeatedly objected to this term and several journalistic outlets have banned or restricted its use in their articles. Nevertheless, the term has stuck and entered widespread usage, and seems unlikely to go away anytime soon.