The snow pea (Pisum sativum var. macrocarpon) is a legume, more specifically a variety of pea eaten whole in its pod while still unripe. It is one of the earliest-known cultivated plants, with evidence of having been cultivated in a region that is now along the Thailand-Burma border, 12,000 years ago.
It is speculated that the name comes from the whitish tint reflected from the pods. It also may come because of their tendency to grow at the end of winter, just before the last spring freeze. They can be covered with snow - hence the name - during these times, but still keep growing well.
As with all legumes, snow peas host beneficial bacteria, rhizobia, that fixes nitrogen in the soil -- this is called a mutualistic relationship -- and are therefore a useful companion plant, especially useful to grow intercropped with green, leafy vegetables that benefit from high nitrogen content in their soil.
The green shoots can also be cut and served as a vegetable as is done in Chinese cooking. Especially stir-fried with garlic or shellfish such as crab.