The barong tagalog (barong for short, pronounced BAR-ongg), as shm00nkie p00nks says, is the traditional Filipino formal male costume
worn for the same social occasions as the Western three-piece suit. These are
coats buttoned down the front, and decorated with embroidery (typically native designs,
although companies sometimes have them custom made with the corporate logo). They come
with either a traditional European-style pointed collar or a chinese-style Mandarin collar. They are
not tucked in, and typically have vertical slits on the sides to allow easy access to
your pants' side pockets.
The piña barong tagalogs are the most expensive (and most delicate), woven from tough pineapple leaf fibers. These are typically translucent (and a bit stiff and scratchy)
so a long-sleeved undershirt is mandatory. The traditional undershirt is the
camisa de chino, a chinese-style open-necked cotton shirt with two or three buttons
at the collar. These barongs have embroidery all over the front and sleeves (with a
few with embroidered collars).
Piña barongs are only worn on formal occasions (weddings, baptisms, etc) but are too uncomfortable and delicate (they tear easily) for daily use. Some of the best piña cloth comes from Lumban, Laguna, where you can also have a barong made, typically ranging from PhP5,000 up to Php20,000 (about US$100 to US$400), depending on extent and intricacy of the embroidery and quality of the cloth.
Most people wear the cheaper barongs made from jusi cloth (abaca fiber or banana-leaf fiber, woven with silk), which are not as expensive as piña, although these need the same care and maintenance (only steam-press, no ironing, gentle cycle).
Casual barongs are made from plain cotton or silk/cotton weaves - these are opaque, softer coats suitable for daily office wear and come in long- and short-sleeved varieties.
Since this coat is not translucent, you can wear your choice of undershirt or tank top (I prefer short-sleeved v-necks) or not wear anything under them on a hot day. The embroidery
on these cheaper barongs are typically limited to two vertical strips, on each side of the chest, extending down to the waist. Unlike the piña cloth, which only comes in cream or white, cotton comes in a variety of colors, with the most common being green, brown, gray, or peach. These typically cost about PhP500-PhP2000 (about US$10-US$40) locally.
The barong tagalog is a descendant of the 16th century baro, worn by natives of lowland Luzon (the Tagalog, or taga-ilog, "dwellers by the river"),
which was a long-sleeved doublet made from rough cotton (canga). With the Spanish
conquest of the islands, a European-style collar was added (typically trimmed with
lace). During the 1800s, the collar shrunk and the lace was replaced with the elaborate
embroidery found on modern barongs.
It is said that the Spanish authorities required that barongs worn to formal occasions be translucent, in order to keep the natives from concealing bolos (machetes) or other weapons beneath their coats.
The barong, in several forms, was popularized by a succession of Philippine nationalists (including Presidents Quezon, Magsaysay, and Marcos) throughout the 20th century. Currently, all male government employees are required to wear a barong on Mondays, and most officials and businessmen wear them every day for meetings and other social functions.
Some details taken from www.mybarong.com.