In almost all scientific communication and labeling practices, however, a third identification tag, the family name, is added - e.g. Quercus alba (Fagaceae). This greatly increases the utility and comprehensibility of the naming system.
The binomial system itself evolved from a fundamental human instinct to recognize categories of things, and specific types within those categories. Before Linnaeus established the formalized latin system that gave us Quercus alba, there were "white oaks, red oaks, etc. (and the equivalent in various other languages), just as there were John Smith, William Smith, etc. Referring to just a "white" or "John," or "William," doesn't tell us much at all.
The binomial gives some context to a name, and helps us interpret new information. If someone describes a new species, Quercus antarctica (hypothetical) for example, we immediately know that it is another species of oak. We can predict that it will be a woody tree or shrub with simple leaves, and that it produces acorns. The family name adds another layer of recognition and predictiveness.
Suppose, for example, someone comes into the room raving about the spectacular specimen of Trigonobalanus doichangensis she'd seen at a botanical garden in Singapore. I myself would have stared blankly at her, having no idea what that gibberish stood for. But then she tells me that Trigonobalanus is a genus in the family Fagaceae. A big light bulb turns on in my head. Fagaceae is the family to which Quercus belongs, along with Castanea (chestnuts), Fagus (beeches), and several other genera. Suddenly I have an approximate idea of what this plant is.
The family name is therefore extremely valuable for recognizing, characterizing, identifying, labeling, storing, retrieving, and providing relationship context for plant specimens. Sometimes it is of more value than the genus name for providing a rough idea of what a plant is and where it fits in relationship to other plants, as in the Trigonobalanus example above. This requires, of course, some knowledge of plant families. Learning the characteristics of families is a routine part of studying plant taxonomy, but will also be highly useful to anyone with an interest in plants. Even the use of common names like "the orchid family" or the "iris family," etc., will be helpful when communicating with a lay audience.
The taxonomic system is a hierarchy of taxonomic categories, or taxa. Genus and family are two levels of taxa. Theoretically, we could also append the names of higher categories, like orders, classes, phyla, etc. You will find those in textbooks, but for everyday use, they would amount to information overload. We can refer casually to important higher categories, like angiosperms, gymnosperms, green algae, etc., without really worrying about their technical names or their rank (their level within the hierarchy).
Familly names for plants have been standardized with the "aceae" ending, which is attached to the name of the first named genus in the family The Asteraceae (sunflowers, etc.) gets its name from the genus Aster. So you'll know when you're seeing a family name. Some older names were different, ending in "ae," and using a descriptive term instead of a genus name as the base. The old name for the Asteraceae was the "Compositae," referring to the composite or compound nature of the flower heads. You will still see these type of names in the older literature. Some of the other common ones are "Palmae" (for Arecaceae), "Gramineae" (for Poaceae), "Leguminosae" (for Fabaceae), "Labiatae" (for Lamiaceae), "Crucferae" (for Brassicaceae) and "Umbelliferae" (for Apiaceae).
The point(s) of these remarks are several:
2. When identifying plants, recognizing the family narrows down your search and allows you to skip over what is usually the most difficult part of a taxonomic key.
3. Referencing the plant family when writing or talking about plants puts them into a context of relationship. The taxonomic system is not an arbitrary set of names, but reflects the natural evolutionary relationships among plants.