A:
The word weed refers to any plant that interferes with the desired use of a field or other tract of land. Weeds may be choking out crops or interfering with a desired habitat, for example. In a cropping system like row crop farming, weeds compete for water and soil nutrients with the planted seed or fruit, and they can also impede harvesting or processing. For that reason, weeds are generally considered undesirable, even when they have positive attributes. For instance, alfalfa (Medicago sativa) is a desirable forage and hay crop, but it's considered a weed in a vegetable garden.
Some weeds that have been introduced to a region from another country or continent are so aggressive and disruptive that they are considered invasive species and subject to regional or nationwide coordinated eradication efforts. These plants, usually grasses, are often called "noxious weeds" or "non-native species."
If you're trying to identify a specific weed, the best resource is probably a weed identification manual for your area. Look for a description that includes black-and-white drawings, world distribution maps and detailed information on the plant, such as its biology, life cycle and effects on different crops.
You can also try using an app like Google Lens, which uses your smartphone camera to help you identify objects and find information about them. The University of Illinois offers a weed identification database that lets you narrow your options by checking a few features, such as flower color, stem length and arrangement, or whether it has leaves with swollen sheaths at the base where they attach to the stem.