Didgeridoo information and resources
Simple answer - a hollow tube! The other answer takes a bit longer.
Looking around the internet, you will read that didgeridoos are predominantly Eucalyptus wood, hollowed by termites, and played by native (often referred to as aboriginal) Australians. This is both true and false.
The didgeridoo is a musical instrument that comes under the category of 'aerophone' - namely, a hollow tube played by applying the lips to one end, creating a seal, then blowing air out of the mouth, causing the lips to vibrate. Sound travels down the bore of the tube, exiting the other end. This principal has been applied by numerous cultures all over the world, apparently independently from each other.
In Ireland there is an instrument called the 'Dordeseal'. The instrument is of metal construction, a curved tube of metal not dissimilar to the shape of a tenor saxophone (but without the knobbly bits). I believe the original has been dated as several thousand years old.
Throughout Asia, in numerous countries, long hollow horns have been used, often in a religious capacity. The instruments mainly made from metal, though wooden examples can be found. Evidence of their use goes back many centuries.
In Switzerland, the national instrument is the Alphorn. This another example of a long hollow tube, in this case made from wood. I am not sure as to how far back there is evidence of this instrument, however I am certain it was prior to the discovery of Australia.
Which leads nicely onto Australia. The native Australians, use an instrument in their culture called the didgeridoo or similar by non-indigenous people. With many clans/tribes and many different tongues there are numerous genuine native names for the didgeridoo. The predominant or best known name is 'YIDAKI', and originates from North East Arnhamland (NEAL), the other known as 'Mago' which originates from Western Arnhamland (WAL). The difference in very basic terms is that the Yidaki is long with a thin or norrow bore, whereas the Mago is shorter with a wider bore. In essence, the didgeridoo is the modern, western equivalent to Yidaki/Mago.
When Australia was 'discovered' and the didgeridoo heard for the first time, it was coined 'Didgeridoo' to resemble the sound the instrument made. The way I see it is that a didgeridoo is an instrument of no specific origin but reflects a western interpretation or response to the aspects of many cultures. A real didgeridoo in the true sense is actually a Yidaki or Mago and can only be produced/made by the people from the territories mentioned above.
There are over 500 species of Eucalyptus, many of which can be and are used to make didgeridoos, though not all types are suitable (such as Eucalyptus Grandis - which, though strong, splits and cracks really badly). Many so-called authentic didgeridoo's that are sold (Australia exports and estimated 500,000) didges a year (mainly through tourism), are not genuine at all. Approximately 25% of trees in Australia are hollow to some degree. Unethical businesses cut down an acre of trees knowing that a quarter of them will provide some revenue - thus causing environmental harm (numerous species of animal and bird are reliant on the termites and hollow trees - an integral and vital part of the ecosystem. These are then sold as genuine and authentic. Not only is this rubbish, but they are not made by people who know what they are doing so the instruments are inferior quality. Always check the source and provenance when considering buying a didge - many are not what they say on the label.
Off the soap box now!
Outside of Australia, the didgeridoo does not have the strong and vibrant cultural heritage. The music does not have the history with different playing techniques and styles developing as a result. Didges have been made from lots of materials - wood, metal, clay, plastic, to name a few. Wooden construction can be extremely inventive, creative and varied, ranging from didges made with matchsticks, to didges made from cactus, didjes made by drilling the wood out, by cutting the wood in two, scooping out the excess then sticking the two halves back together again (this is the method I use).
In terms of playing, there are quite a number of styles and techniques. Traditionally, the best known are NEAL and WEAL techniques. Both are extremely complex and difficult to play - after all the technique has been developed and built over many centuries. Outside of traditional playing there is everything from dance, trance, drum and bass to ambient and medative. Music can be found in most genres where someone has mixed in the didge. Though the techniques may not be as structured and controlled as NEAL and WAL, the variety of sounds can be just as wide.
Outside of NEAL and WAL there is no right or wrong way of playing and comes down to personal preference, it is what you like the sound of that matters, not what others think is the 'proper' way of playing. Sorry soap box again.
I have been brief in my explanation, but there is more information around this site including links to other sites if you fancy some further reading! To finish off, here are all the different spellings and miss-spellings I have come across so far:
didgeridoo, didgeridu, didjeridu, didjeridoo, didgerido, didge, didg, didje, didj, didgereedoo, didgereedo, didgereedu, didjerido, didjereedoo, didjereedo, didjereedu, digeridoo, digerido, digeridu, digereedoo, digereedo, digereedu, dige, dijeridoo, dijerido, dijeridu, dijereedoo, dijereedo, dijereedu, dije, dij, dijj, didgeredoo, didgeredu, dijje, dijjidu.