The term weed is generally used to refer to any plant growing where it is not wanted. They are not seeded or cultivated intentionally but continue to come back each time they are removed from the setting where they are considered undesirable.
There is no real weed classification in the taxonomy of plants. A plant can be a weed in one setting where it is not wanted, yet it can be intentionally cultivated in other settings where it is not considered a weed.
Plants are often classified as weeds because they are deemed by humans as detracting from the aesthetics of certain settings as is the case in gardens, lawns, and other landscaping. They can make certain settings dangerous for humans because they cause nonuniformity such as on a football or soccer field where they can lead to injury like twisted ankles. Certain invasive plants or weeds can take over a setting and kill native plants in the area.
Weeds can, however, be beneficial to humans in certain settings. For example, they are often the only types of plants that will grow in areas where more desirable plants will not grow such as areas where the soil is compacted, areas that are too moist, or where the soil’s pH is too acidic or alkaline. Weeds often prevent soil from eroding because the grow quickly in disturbed areas. They often attract wildlife by providing a source of food.