The Praetorian Guard (sometimes Prętorian Guard) (in Latin: praetoriani) comprised a special force of bodyguards used by Roman emperorss. Before them it was used by warlords, back at least to the Scipio family -- around 275 BC.
Marcus Antonius and Augustus Caesar (Octavianus) assembled special praetorian troops, the praetoria cohors. They consisted of both infantry and cavalry. Octavian gave the prętorians a permanent presence, forming nine praetorian infantry cohorts of 1,000 men each and a number of cavalry units (turma) of 30 men.
The praetorian guards had higher wages and less onerous duties than other soldiers. In the beginning they were recruited solely from Italy but, as time went on, they also originated in other provinces. They also received gifts of money called Donativum from the emperors.
The special position of the Praetorians made them become a power in their own right in the Roman state, and their prefect, praefectus praetorio, soon became one of the more powerful men in this society. The emperors tried to flatter and control the praetorians, but they staged many coup d'états and contributed to a rapid rate of turnover in the imperial succession. The praetorians thus came to destabilize the Roman state, contrary to their purpose.
In 193, the praetorians even arranged an auction for the throne, an auction won by Didius Julianus. After seizing power and executing Didius, Septimius Severus cashiered the current praetorian units and established an entirely new guard of 50,000 men from soldiers loyal to himself.
The final act of the Praetorians in imperial history occurred when the Caesar Flavius Valerius Severus, following the orders of Galerius, attempted to disband them in 308, but they revolted, elevating Maxentius as their Emperor. Following his victory in the Battle of Milvian Bridge, Constantine the Great definitively disbanded the Praetorian Guard.
In common language, the phrase "praetorian guards" designates an exclusive group attached to powerful people, such as Adolf Hitler's SS troops. However, the term is used in non-military contexts: for example, a corporate officer or politician may have a small group of fanatical followers whom a journalist may describe as a "praetorian guard".