Miles gloriosus is a standard character-type found in many plays. He first appeared in Greek comedy, where he was known as alazôn. He passed on into Roman theater, where he picked up his current name, and then into Renaissance plays. He is a well-know and oft used archetypal braggart-soldier. These days he is rarely given his Latin name, as few would recognize it, but he still appears.
He is a cowardly parasite, prone to bragging and swashbuckling, and may be victimized and mocked by the other characters.
You may have seen him as:
Ralph Roister Doister
Captain Bobadil
Quintiliano
Sir John Falstaff
Don Armado de Adriano
Parolles
Ancient Pistol
The Cowardly Lion
(Brave) Sir Robin (the one who ran away).