Some weeds are plants. They are considered as weed because they are invasive and grow aggressively and in the wrong place. They grow every where such as lawns, farm field, parks, driveways, pavements, all kind of surfaces whether soft or hard. In fact I saw the weeds growing also on roof and walls of many old buildings and houses. Due to these reasons these plants are unwanted by everyone like gardeners, farmers, homeowners etc.
Many plants which are considered as weeds also intentionally grown by people who want them to make medicines or any herbal remedies or to eat because some weeds are also edibles but, another species in the same genus might be poisonous weeds.
Weeds are seriously unwanted in a specific locations because of their interference with growth of cultivated plants like fruits and vegetables and fibre production in agriculture where are risks of loosing or reducing the crops. Weeds are also seriously unwanted in cattle farms because they might be harmful to animals and cause serious loss.
There are some weeds information I have listed below which I have studied about. These weeds are considered as weed because they always in wrong place , but they are in fact wild flowers.