A typical chillum (or chilam or Shillum) is a straight conical pipe. Traditionally, chillums are made of clay, stone or wood and have been used by the Sadhus (Holy Men) in India and by Hindu monks in the Himalayan area since at least the 18th century to smoke opium, cannabis and other narcotics or herbs, but there have been finds of old chillums in South Africa as well, so the true origin of the chillum is unknown.
In India, smoking, or even ownership of a chillum was traditionally limited to male users and was mainly practiced in the rural areas of northern India. When they lit the chillum, they would touch their forehead with the chillum and then say: “Boom Shankar”. This was to invite Shiva and the other gods to come smoke the chillum with them.
A chillum was usually passed around and this made it especially important for the early stoner and hippie movement and their feeling of being "one" or part of a "family" in the 60s. At the same time, stoners returning from trips to the Indian sub-continent brought chillums back home as souvenirs (ok, not only for looking at them we assume) and the relatively easy way of smoking and cleaning added to their popularity. Side-note: Allegedly, the natives of America passed their chillums around anti-clockwise; it is said they wiped their - pardon me - ass with their left hand and would never touch a chillum with their left hand, so they would always pass it to the one to their right.
Today, or better said since the 1990s, the use of chillums is less popular due to all the high-end smoking paraphernalia and there are actually more chillums produced in Italy than in India, but older consumers of (not only) the hippie scene still stick to this archaic and very direct way of smoking cannabis and the fact that the smoking device has to be passed around to share the experience. Even the Rastafarians use chillums in their rituals - and miss the correct date by just one day with one of them! They celebrate their "Grounation Day" on April, 21st - D'OH! No 420, but still a good day to have a jointly smoke. But I digress...
There are crazy variations of chillums - people are inventive and carve disposable organic carrot or radish chillums (cool idea by the way, it adds some interesting flavor to the smoke) and some people make an art out of chillum creation. Darrel "Pipeman" Mortimer for example, a street artist from San Francisco made almost 10,000 chillums since 1970 and personally signed and sold them - ain't that dedication?
Many people in the stoner community call the down-stem of a bong also a "chillum" - from our knowing it is the most widely used term in stoner slang for this particular item. Bong "chillums" are usually made of glass or some kind of stainless metal.