Rhubarb is a herbaceous perennial, species Rheum rhabarbarum in the family Polygonaceae. They grow from short, thick rhizomes and have large triangular leaves with fleshy petioles ("leaf stalks").
The domesticated form Rheum x hybridum is grown for its fleshy leaf stalks. These are cooked and provide an acidic (tart) but fruity flavour.
Technically rhubarb stalks are a vegetables (they are a non-reproductive part of the plant) but they are usually treated as if they are fruits for culinary purposes. Many legal jurisdictions, including the US, class rhubarb as a fruit for regulatory purposes.
Rhubarb prefers cool, wet conditions, such as the northern US and Northern England.
Rhubarb can be a strong laxative and has been used as such in China for at least 5000 years. It also appears in some European and Arabic medieval prescriptions.
Many parts of the rhubarb plant are toxic. Do not eat the leaves, which contain high levels of oxalic acid. The stalks contain a small amount of oxalic acid and some malic acid - these produce the tart flavor but are not present at toxic levels.