Known by a small army of common names--probably best-known as Lemon Clover but also referred to as Yellow Woodsorrel, American Woodsorrel, Bitter Clover, Lemonade Clover, Common Yellow Oxalis, and Wild Oxalis. It is indeed an Oxalis and is known scientifically as Oxalis stricta--and together with its nearly identical cousin Creeping Oxalis (Oxalis corniculata) these two wild plants are often the first-discovered edibles by youngsters. They have a pleasant, citrusy flavor and can be enjoyed raw, candied, steamed, sauteed or added to soups. Every part of the plant has some edibility, including the tough middle stems and roots which can add acidity and Vitamin C to herbal teas. A non-lemon substitute for lemonade (made for those intolerant or allergic to citrus) is made from this plant.

Oxalis stricta and cornuculata identification traits:
-A small plant, typically not exceeding six or seven inches high. O. stricta grows like a miniature of a shrub, in little bushy clumps, and O. cornuculata instead grows as a creeping plant, often following the cracks in concrete and bursting out its leaves and flowers from normally very inhospitable crevices.
-The leaves, a bright yellow-green, grow in trios and are heart-shaped. It strongly resembles a shamrock, and the leaves have a few visible veins running towards the top of the heart from a central point. The edges of the leaves are smooth, not ragged or scalloped.
-Delicate and obvious yellow flowers, growing singly from a sepal and with five petals.
-The seed pods, appearing once the flowers have dried and disappeared, are tall, pointed and have five sides to them. Once they dry, they pop open spectacularly and fling the tiny blackish seeds into new soil, far from the parent plant.
-Distinctive lemon-like flavor and odor upon tasting or crushing the leaves.

One potential look-alike could be the Black Medic plant. It is also edible, but contains compounds that could cause early termination of pregnancy or birth defects, so it I'd recommend learning it in order to avoid it if you happen to be preggers. :D

Black medic:
-Is in the alfalfa family, and has similar clover-like leaves. These leaves are not heart shaped but oval, and often have scalloped or ridged edges.
-The leaves are compound, branching from a small stem rather than all growing from a single point on that stem.
-The flowers are compound, with many small yellow flowers growing in a cluster at the end of a small stem. These flowers are much smaller in proportion to the plant than with the wood-sorrel, and are less showy.
-It does not have a very distinct lemony taste, but rather tastes spinachy and green.
-Generally grows as a runner, never upright.

:D The lemon clover is available in early spring at its best quality. As I mentioned, all parts are edible! The leaves, seed pods, tender stems, and flowers are the most desirable parts, and are nearly unrivaled in Vitamin C content. Of all the small plants considered "weeds" by most, these are perhaps one of the most deserving of more active cultivation and appreciation. Historically it was even used to treat scurvy (severe Vitamin C deficiency) and sore throats!

:D But as always, be sure you know exactly what fungi or plant you have before enjoying. When in doubt, throw it out!