There is a good chance that you have seen a plant of Trifolium repens, or white clover, today. Originally from Europe, northern Africa,and Asia, white clover (also called Dutch clover) is widely naturalized in most parts of the world. It is commonly found in lawns, parks, gardens, roadsides, rangelands, and in most types of disturbed sites.
As far as weeds go, it is a rather useful one. Trifolium repens (pdf) fixes nitrogen, and is highly-resistant to trampling, making it suitable for high-traffic areas such as lawns. It is also useful for farm lanes, where it prevents soil compaction from farm equipment. It is an excellent, high-protein forage crop for livestock, and can also be boiled lightly to make it digestible to humans. Trifolium repens is considered one of the best honey plants by apiarists–you may have tasted clover honey, which has a mild, floral flavour.
As a child, perhaps you spent time crawling through the grass, searching for four-leaved clovers. According to the science writer David Bradley though, four-leaved clovers are only considered lucky if you find them by accident (actually, Bradley quite adamantly asserts that there is no luck involved, only a random mutation that occurs about once in every 10,000 specimens). Therefore, 99.99% of the time, Trifolium repens leaves are trifoliate (have three leaflets). They are alternately arranged and borne on long, curving stalks. The individual leaflets are oval, egg-shaped, or heart-shaped and are 10-55 mm long by 6-30 mm wide. The adaxial (upper) surface of each leaf has a distinctive white chevron (upside-down “v”) marking.
The white clover inflorescence is a globose (spherical) head borne on a long stem. Each head has 20-100 white to pinkish florets, borne on a short stalk (pedicel). Bruce’s photo shows that each of those florets resembles a typical pea flower–a feature I had not noticed, but which makes a lot of sense, since Trifolium is in the pea family, Fabaceae. When fertilized, the florets produce tiny pods (4-6 mm long), each containing 3-4 seeds that remain hidden inside the old flower parts. The seeds are dispersed by livestock, earthworms, birds, and humans.