Horticulture is the art and science of cultivating Herbaceous plants including fruits, vegetables, flowers, ornamental plants, herbs and spices, and many other plants used directly by humans. 

Horticulturists work with  landscape architects, botanists, farm and orchard managers, grocers, cultural preservatist for historic gardens and arboretums, media specialists, and artists.

Horticulture is promoted in the US by many organizations, including the American Society for Horticulture Science (www.ashs.org) and the American Horticultural Society (www.ahs.org) and throughout the world by the International Society for Horticultural Science (www.ishs.org).

Societies Preserving Historic and Culturally Important Gardens, Landscapes, Arboretums, and Forests
National Parks
The Cultural Landscape Foundation
City and Regional Guides
The Garden Conservancy
National Register of Historic Places

The National Trust for Historic Preservation

Herbaceous plants fit roughly into 3 categories, namely annuals, biennials, and perennials.
Annuals are plants that complete their growing cycle in a single year or season. They germinate, grow vegetatively, flower, set seed, and then die. Many plants grown as annuals in temperate climates may actually be "tropical perennials".
Biennials are similar to annuals, except that they do their vegetative growing one year, then flower, set seed, and die the following year.
Perennials are plants that live year after year, going through regular cycles of vegetative growth, flowering, setting seed, and then returning to vegetative growth (usually with periods of dormancy in between). Woody plants such as trees, shrubs, and vines are also perennials.