Weed

Weed, term applied to any plant that grows where it is not wanted. A weed is usually characterized by rapid growth, and it typically replaces other, more desirable plants. Some plants, such as crabgrass, are considered weeds everywhere they grow, but many plants are considered weeds in some regions and not in others. For example, the shrub lantana is raised as an ornamental plant in many parts of the United States, but has become a serious pest in Hawaii.
Weed could damage in some way:

• Competing with them for sunlight, water, and mineral nutrients
• Parasites, grow directly on other plants.
• As hosts for disease-causing organisms. For example, some of the fungal diseases that
• infect food crops spend part of their life cycle on a weed that typically grows near the crop.

Even most of weed are harmful, some weed can provide benefit like, serve as safe and nutritious forage for grazing animals, prevent or retard soil erosion in open pastures and stabilize the thin, fragile soils of tropical farmlands, conserve water in semi-arid and Mediterranean climates by enhancing water storage, increasing shade, and breaking up soil with their root growth.

Weed Control

• Hand pulling, the underground root must be pull out to avoid the weed back. Even tiresome, this most effective method.
• Mulching, applying a layer of leaves, straw, or other organic mulching material over the weeds. Good practise when the root system of your plant is sensitive.
• Herbicide, using chemical to kill weed.
  

Source: Encarta