The term weed is often used to refer to marijuana, but it can also describe other plants that interfere with crops or pastures. These plants may be non-native to the area and could disrupt ecosystems, or they might have a number of traits that make them a nuisance, such as high tolerance to stressors like low or excessive levels of certain nutrients in soil; drought, waterlogging, or temperature extremes; or repeated grazing or mowing by livestock or tillage.
Many weeds are easy to identify in the flowering phase, when they show the most defining characteristics. But growers often want to know what they’re dealing with in their earlier growth stages, especially as seedlings and sprouts, so that they can develop strategies to control them before they become more troublesome.
When a weed has just emerged from the ground, for example, it can tell you whether it’s a true seedling (easy to dig up, with fine roots and remains of the seed coat or seed head visible), or if it’s a shoot from a root, rhizome, or other perennial underground structure (harder to dig up, with a lot of coarse, long roots). Direct comparison between a specimen and photos or written descriptions in a plant identification manual can confirm the correct ID.
The University of Illinois has an excellent weed database, which you can browse by common names or Latin names and narrow down by features like leaf width and shape or floral color. Virginia Tech and Clemson have similar tools.