In Portuguese, these nuts are not called "Castanhas do Brasil" which would be a direct translation of the English name. Rather, they are known as "Castanhas do Pará" taking their name from the northern Brazilian state of Pará, where they were first commecially harvested.
The Brazil Nut tree (Bertholletia excelsa) is native to the Amazonian Rain Forest, and can be found from Bolivia to the Guianas. It is one of the largest trees in the rain forest, reaching heights of up to 100- 150 ft. (30 - 45 m.) and normally lives for up to 500 years, although some specific trees are older than 1,000 year. The International Union for Conservation of Nature (IUCN) classifies the Brazil Nut tree's status as "vulnerable" as many of its habitats have fallen victim to deforestation.
The Brazil Nut has a very interesting, and extremely complicated, means of reproduction, which requires a specific species of orchid and a "long-tongued orchid bee." Wikipedia outlines the process like this:
The Brazil nut tree's yellow flowers contain very sweet nectar and can only be pollinated by an insect strong enough to lift the coiled hood on the flower and with tongues long enough to negotiate the complex coiled flower. For this reason, the Brazil nut's reproduction depends on the presence of the orchid Coryanthes vasquezii, which does not grow on the Brazil nut tree itself. The orchids produce a scent that attracts small male long-tongued orchid bees (Euglossa spp), as the male bees need that scent to attract females. The large female long-tongued orchid bee pollinates the Brazil nut tree. Without the orchid, the bees do not mate, and therefore the lack of bees means the fruit does not get pollinated.
With such a complicated system of reproduction, it's fortunate that each tree can live up to a thousand years - the chance of all required parties showing up at the same time and same place for reproducing is likely rather small.
Brazil Nuts are high in both calories and protein (18% protein by weight). It is also the richest dietary source of the mineral selenium, which has lead some researchers to suggest inclusion of Brazil Nuts in one's diet, due to selenium's significant powers in preventing certain cancers, including breast cancer and prostate cancer. The shell of this nut, however, contain alfatoxins which can lead to liver cancer. This has caused the EU to impose import restrictions on "in-the-shell" Brazil Nuts.
In its native habitat, the Brazil Nut is eaten by the local population raw, roasted, ground into flour, and in both savory and sweet dishes, including particularly, a delicious ice cream. Outside Amazonia, the bulk of exported nuts are eaten roasted and salted.