Halloween aka Hallowe'en (a contraction of Hallows' Even or Hallows' Evening), also known as Allhalloween, All Hallows' Eve, or All Saints' Eve, is a cultural celebration observed in a number of countries on 31 October, the eve of the Western Christian feast of All Hallows' Day.

A traditional Irish Halloween turnip (rutabaga) lantern on display in the Museum of Country Life, Ireland. image: Rannpháirtí anaithnid/wikipedia
It begins the three-day observance of Allhallowtide, the time in the liturgical year dedicated to remembering the dead, including saints (hallows), martyrs, and all the faithful departed.
Origins
It is widely believed that many Halloween traditions originated from ancient Celtic harvest festivals, particularly the Gaelic festival Samhain; that such festivals may have had pagan roots; and that Samhain itself was Christianized as Halloween by the early Church. Some believe, however, that Halloween began solely as a Christian holiday, separate from ancient festivals like Samhain.
Activities
Halloween activities include trick-or-treating (or the related guising/from disguising but really just means dressing up), attending Halloween costume parties, carving pumpkins into jack-o'-lanterns, lighting bonfires, apple bobbing, divination games, playing pranks, visiting haunted attractions, telling scary stories, and watching horror films.

Jack-o’-lantern carving. image:wikipedia
In many parts of the world, the Christian religious observances of All Hallows' Eve, including attending church services and lighting candles on the graves of the dead, remain popular, although elsewhere it is a more commercial and secular celebration. Some Christians historically abstained from meat on All Hallows' Eve, a tradition reflected in the eating of certain vegetarian foods on this vigil day, including apples, potato pancakes, and soul cakes.
The story of Jack
There is a popular Irish Christian folktale associated with the jack-o'-lantern, which in folklore is said to represent a "soul who has been denied entry into both heaven and hell"
On route home after a night's drinking, Jack encounters the Devil and tricks him into climbing a tree. A quick-thinking Jack etches the sign of the cross into the bark, thus trapping the Devil. Jack strikes a bargain that Satan can never claim his soul. After a life of sin, drink, and mendacity, Jack is refused entry to heaven when he dies. Keeping his promise, the Devil refuses to let Jack into hell and throws a live coal straight from the fires of hell at him. It was a cold night, so Jack places the coal in a hollowed out turnip to stop it from going out, since which time Jack and his lantern have been roaming looking for a place to rest
How it spread to the US
Lesley Bannatyne and Cindy Ott both write that Anglican colonists in the Southern United States and Catholic colonists in Maryland "recognized All Hallow's Eve in their church calendars”, although the Puritans of New England maintained strong opposition to the holiday, along with other traditional celebrations of the established Church, including Christmas.
Almanacs of the late 18th and early 19th century give no indication that Halloween was widely celebrated in North America.
It was not until mass Irish and Scottish immigration in the 19th century that Halloween became a major holiday in North America.
Confined to the immigrant communities during the mid-19th century, it was gradually assimilated into mainstream society and by the first decade of the 20th century it was being celebrated coast to coast by people of all social, racial and religious backgrounds.
"In Cajun areas, a nocturnal Mass was said in cemeteries on Halloween night. Candles that had been blessed were placed on graves, and families sometimes spent the entire night at the graveside”.
The yearly New York Halloween Parade, begun in 1974 by puppeteer and mask maker Ralph Lee of Greenwich Village, is the world's largest Halloween parade and one of America's only major nighttime parades (along with Portland's Starlight Parade), attracting more than 60,000 costumed participants, two million spectators, and a worldwide television audience of over 100 million.
Commentary
Why it’s not as popular in the Australia?
Probably partly because of the late adoption in the US. The fact that it was adopted so late in US and Australia wanting to strive for greater independence from US influence after WWII . Though in more recent years (past 15-20 years perhaps) through greater internet access and social media some limited aspects are celebrated. Also it is the beginning of summer rather than winter.
Source adapted from: Wikipedia contributors. (2018, October 31). Halloween. In Wikipedia, The Free Encyclopedia. Retrieved 05:31, October 31, 2018, from https://en.wikipedia.org/w/index.php?title=Halloween&oldid=866570858