Oxalidaceae is sometimes known as the woodsorrel family, although it includes a number of plants not referred to as woodsorrel, including the Biophytum sensitivum, and, by some accounts, the Starfruit.
More commonly, woodsorrel refers to members of the Genus Oxalis. With over 800 species appearing on nearly every continent, there is a wide range in appearance, and not all species are commonly known as woodsorrel. However, most species are recognizable as being related, and the great majority are referred to as either being some sort of woodsorrel, sorrel, or shamrock (often 'false shamrock'). Despite these names, woodsorrel is neither sorrel nor clover, although it usually tastes like sorrel, and its leaves look like clover leaves.
Woodsorrel is usually easily recognizable, having sets of (usually) three heart-shaped leaves on each stem, looking very much like clover. They can be easily distinguished from clover by their flower; while clover has flower heads made up of many small pin-point blossoms, woodsorrel has more traditional, trumpet-shaped flowers, small and sometimes patterned or striped, but most often in a single solid color. They may come in white, pink, red, or yellow; yellow is particularly common in the 'weed' woodsorrels that appear in lawns and meadows.
Woodsorrel is also quite distinctive in taste, containing oxalic acid, the same chemical that gives true sorrel its distinctive taste. Generally all parts of the plant can be eaten, although most people limit themselves to the stems and leaves. Sorrel tends to be crisp and refreshing, with a light sour flavor that as a child I compared to lemon; others have compared it to rhubarb, while others simply call it 'tangy'. In parts of the US woodsorrel may be referred to a 'sours'. Many cultures around the world have added woodsorrel to salads, soups, desserts, teas, or eaten it plain. It is widely said to quench thirst, and various species have medicinal uses. A species commonly known as Oca (Oxalis tuberosa) was cultivated in the Andes for its tuberous roots, and has since been introduced to Europe and then New Zealand (It became quite popular in New Zealand, and is sometimes called the New Zealand Yam).
It is important to note that oxalic acid is slightly toxic, and while it is generally safe in small doses, it should not be eaten in large quantities or by people with impaired kidney function.
Woodsorrel is quite pretty, and is often sold as a decorative plant. There are species that grow well in shade and others that do better in sunlight; many do well in poor soils, and some are healthy enough that they are seen as weeds. Despite their resemblance to clover, they do not fix nitrogen in the soil, and they are generally not a good forage crop. They are not generally invasive enough to be a problem in garden plots, but they will keep coming back.
You may know woodsorrel as Oxalis, Shamrock, Yellow-sorrel, Pink-sorrel, English weed, Soursob, Sourgrass, Soursop, Sours, Lemon Clover, Pickle Plant, Oca, Oka, New Zealand Yam, or any number of other names, most of them containing the words 'sorrel', 'shamrock', or 'oxalis'.
Woodsorrel is generally written as one word in America, and two words (either 'wood sorrel' or 'wood-sorrel') in Europe and the UK.