"Kelp" is the general term for 124 species of brown algae that are in the Order Laminariales. While kelp look like plants, they are classified in the Kingdom Chromista.
The kelp plant itself is made up of three parts: the blade (the leaf-like structure), the stipe (the stem-like structure), and the holdfast (root-like structure), which grips a substrate and anchors the kelp among the waves and currents.
Kelp grows in "forests" in cold waters (usually less than 68 degrees F). Several kelp species can make up one forest. A multitude of marine life lives in and depends upon kelp forests, including fish, invertebrates, marine mammals and birds. The most well-known kelp forests are the forests of giant kelp that grow off the coast of California, which are inhabited by sea otters.
Kelp is not only useful to animals, but to people, too. In fact, you probably even had kelp in your mouth this morning! Kelp contains chemicals called alginates that are used to thicken a number of products (e.g., toothpaste, ice cream). Alginates are also used in pharmaceuticals.
Examples of kelp species: giant kelp, southern kelp, and bull kelp. Giant kelp is, not surprisingly, the largest kelp species, and is capable of growing 2 feet per day in the right conditions, and up to about 200 feet in its lifetime.