This magistracy was originally designed as a sort of third consul and was established in 356 BC for patricians only after they were forced to share the consulship with plebes. This changed by 337 BC when the first plebeian praetor was elected.
Romans were eligible to be praetor at the age of 40. They had imperium with the main functions being administration of civil law in Rome (Praetor Urbanus), military command, judges in courts of law (Praetor Peregrinus created in 246 BC), and finally the governing of provinces.
They also assumed administrative duties of consuls when these were absent from Rome. When there were more than 2 praetors (beyond 197 BC), the additional praetors were generally assigned as governors of Sicily, Sardinia, and the Spanish provinces (and others as province acquisition continued through the late Republic and early Principate).
Like proconsuls, praetors could hold the title of propraetor after their annual term of service and be appointed as provincial governors. They were entitled to 6 lictors.
In the imperial period there were as many as eighteen praetors per year and the office could be held at the age of thirty. The position could be responsible for any number of duties including judicial matters, military command, provincial governorships, grain supplies or road and treasury supervision.