Garlic mustard (Alliaria petiolata) is a fairly common weed or wildflower in Britain and across much of northern Europe, Asia and Africa, and also an invasive species across large swathes of North America. It could be mistaken for a stinging nettle by someone not paying very much attention - its leaves are much the same shape and size, like serrated heart shapes, and they also grow to around a metre tall in hedgerows. The leaves are hairless though, and they grow alternately up the stem rather than in pairs like a stinging nettle. The little white four-petalled flowers are also completely different, as are the erect seedpods that they give way to.
The plant is a member of the mustard family, Brassicaceae, and like its cousins it has the characteristic spiciness of allyl isothiocyanate and similar sulfur compounds - a heat that delivers the same lachrymatory blast to your nasal passages that true mustard does1. This is combined with a flavour redolent of garlic. I find the leaves distinctly bitter - much more so than hairy bittercress, for example - but different people's taste buds detect different things as being bitter, and enough writers make no mention of its bitterness when discussing its taste that I suspect I am in a minority on this. In spite of the bitterness, their complex flavour keeps me coming back for more. The leaves, flowers and seed pods are all edible, although the central stem tends to be a little fibrous. The seeds taste more mustardy, while the leaves carry most of the garlickiness.
The isothiocyanates, with their sinus-assaulting pungency, are among the main reasons people eat any kind of mustard (and much of the reason why cabbage is more interesting than lettuce), but it's thought that their main biological function is to deter animals who might want to eat the plants. Humans have a pretty odd relationship with pain, sometimes - very few other animals go out of their way to eat anything spicy. They also poison other plants, one reason they can be unwelcome as an invasive species2. One reason for the slow-burn effect of this particular kind of spiciness is that the chemicals only transform into their tear-jerking form when the cells of the plant are ruptured, for example by chewing, which allows the enzyme myrosinase to activate them by stripping off an attached glucose molecule1.
There is some evidence that various isothiocyanates have anti-cancer properties3, but it wouldn't be safe to conclude from existing studies that garlic mustard will provide any significant protection against cancer. It is pretty tasty, though - a wild food worth being aware of, if it isn't too bitter for you.
References:
1Gernot Katzer's Spice Pages
2Allelochemicals Isolated from Tissues of the Invasive Weed Garlic Mustard (Alliaria petiolata)
3Google Scholar search for isothiocyanates and cancer