Forb

A forb is a non-woody flowering plant that is not a grass. Since it is non-woody, it is not a shrub or tree either. Thus most wild and garden flowers, herbs and vegetables are forbs.

Forbs do not form a coherent taxonomic group: there are many families and even genera of plants that include forbs along with shrubs or even trees. However the category is useful, and widely used, in two contexts:
*In describing plants for non-specialist purposes, the most useful initial way of breaking down a list is into grasses, forbs, shrubs and trees, because this tells us what basic kind of plant we are looking at. This is the usual way that guides to the plants of a region are organised; see, for example, Wikipedia's List of San Francisco Bay Area wildflowers
*In ecology, the way a plant grows may be more important than the taxon it belongs to. For example, a grazing herbivore is likely to be able to eat a wide range of grasses and forbs, but not to take shrubs or trees even if they belong to the same families. Similarly, in fire ecology, forbs regardless of taxon will pose one kind of danger, and trees a different kind.