A weed is a plant growing where it is not wanted. Weeds rob the desired plants of nutrients and water, may reduce sunlight getting to the crop, may harbor pathogens and insects, and make work with the crop difficult.
Weeds reduce crop yield and reduce the aesthetics of the garden.
Many plant families are dicots. Examples include annual bluegrass, amaranthus, purslane, and purple henbit biennials similar to annulas, especiall in south. vs. Perennials - grow at slower rate than annual weeds, but build strong perennial organ that can withstand adverse weather conditions. Once established the perennial organ is very difficult to control. Cool season weeds have seeds that germinate during the cooler part of the year when the soil is cool. They are frost tolerant and can usually withstand much cold weather.