Bracken, AKA Brake Fern refers to any ferns of the genus Pteridium1. Because most of us are not botanists, 'bracken' may be used as a generic term for any large fern or weed fern, although odds are that your large weed fern is indeed bracken. Bracken are probably most identified with the moors and woods of the United Kingdom, but they can be found around the globe, and are endemic to every continent except Antarctica. The shoots are eaten in many parts of the world, and are a particularly popular food crop in Asian cultures, although there are some health risks associated with eating them.
Bracken are large plants, generally ranging from 1-3 meters in height, and often found in thickets. The prefer well-drained soil, so are common on hillsides and banks, but will grow in nearly any environment aside from the desert. In favored habitats it will form dense thickets which may cause it to be seen as a nuisance. Bracken spread by growing a network of travelling roots deep underground, making them very hard to get rid of.
Bracken is often seen as an invasive weed in pastureland, as it can spread quickly and is harmful to grazing animals, causing thiamine deficiency in animals with one stomach (including humans) and aplastic anemia in ruminants when eaten over a prolonged period of time. It also encourages the presence of the sheep tick, a common carrier of Lyme disease. In the UK it has been traditional to harvest bracken stalks to use as thatch for roofing, and as animal bedding, and then used as fertilizer when thoroughly soiled. As these practices become less and less common, bracken is becoming more invasive.
Bracken spreads quickly, and if not kept under control may take over some ecologies. Natural England has an active bracken control program to prevent bracken from taking over moorland and other desirable habitats. Aside from being poisonous to many animals, bracken releases a toxin into the soil that prevents other plants from growing, particularly tree seedlings. It also competes strongly against heather. Overgrazing and burning on moorlands eliminate the climax community species, and give bracken and coarse grasses an opening to take over, discouraging grazing animals and eventually encouraging wildfires. When bracken stalks die they form a thick moist ground cover, which is idea for the sheep tick. This tick is a health hazard not only to sheep, but also baby birds, deer, blue hares, and grouse.
Bracken sprouts small fiddleheads that are eaten the world over2, although there is some debate over how safe this is. The sprouts have high levels of ptalquiloside, which is both a carcinogen and affects the growth of bone marrow. Soaking and/or cooking the fiddleheads will destroy the ptalquilosides, and it is generally considered safe to eat well-prepared bracken fiddleheads. However, it is believed that the high incidence of stomach cancer in Japan may be due, at least in part, to the frequent consumption of these fiddleheads. As with so many things, consumption in moderation is probably a good idea.
Raw fiddleheads also contain the enzyme thiaminase, which breaks down thiamine (vitamin B1), and can thus cause nutritional deficiency if you eat a lot of them, or alternatively, don't get enough thiamine in your diet. This problem is also solved by cooking the fiddleheads, and so is more often a problem in animals than humans.
The rhizomes are also edible, with the same caveats as the fiddleheads. They have also been used as a source of starch and for brewing beer; as far as I am aware, no studies have been done on the safety of these applications. As these rhizomes are thin and can travel 1.5 to 3 meters underground, few people harvest them these days. It is reportaed that bracken rhyzomes were an important food source for some groups of Native Americans and the New Zealand Maori.
Aside from being a food source, the most common use of bracken these days is as a mulch or compost. Bracken extracts phosphorous and potassium (stored in the form of potash) from the soil, which isn't great for the soil it grows in but is great for the soil it rots in.
Footnotes:
1. There is some debate as to whether there is one species of bracken, Pteridium aquilinum, or 10-12 different species. This is an ongoing problem in the field of biology.
2. Eating abroad? In Japan bracken is known as warabi, and bracken flour is called warabiko (and is used to make dishes like warabimochi). It is worth noting that the Hawaiian warabi is a different type of fern, the Vegetable Fern (Diplazium esculentum). In Korea bracken is called gosari, in China and Taiwan it is known as juécài. In Japan and Korea bracken fiddleheads are most often eaten in the dish bibimbap. In France bracken is called grande fougère or fougère d’aigle.
References:
Overview
Wikipedia
Eating Bracken
Merck Manual
Ecological effects of Bracken in the UK