The easiest way to describe the difference between seaweed and kelp is that kelp is a subcategory of seaweed. Kelp is the subcategory of seaweed consisting of the order laminariales, often described as a brown algae. When in doubt and faced with brown seaweed, just know it’s probably kelp. Kelp makes up the lions share of what we think of as seaweed. However, that is a very crude, probabilistic view.

Kelp does have roots, but they primarily function as anchors for the “plants” to keep them in a given spot. Sidenote: algae are not considered plants as they lack more specialized structures such as functional roots, phyllids, and more. Compared to other species of seaweed, kelp tends to be bigger, thicker, and hardier in general. They also tend to resemble more of what people traditionally think of when thinking of plants despite not being so in a strict sense of the word.