A vine is any plant of genus Vitis (the grape plants) or, by extension, any similar climbing or trailing plant. The word, ultimately derived from Latin vīnea, originally referred exclusively to the grape-bearing plant; the modern extended sense is largely restricted to North American English, which then uses grapevine to refer specifically to the grape-bearing Vitis species. (Conversely, British English tends to use climber to refer to the broader category, including, for example, ivy.)
The remainder of this article uses the term vine in its broader, North American sense.
Certain plants always grow as vines, while a few grow as vines only part of the time. For instance, poison ivy and bittersweet can grow as low shrubs when support is not available, but will become vines when support is available.
A vine is a growth form based on long, flexible stems. This has two purposes. A vine may use rock exposures, other plants, or other supports for growth rather than investing energy in a lot of supportive tissue, enabling the plant to reach sunlight with a minimum investment of energy. This has been a highly-successful growth form for plants such as kudzu and Japanese honeysuckle, both of which are invasive exotics in parts of North America. Conversely, there are some tropical vines that develop skototropism and grow away from the light, a type of negative phototropism.
A climbing habit has evolved independently in several plant families, using many different climbing methods. Some plants climb by twining their stems around a support (e.g., morning glories, Ipomoea species). Others climb by way of adventitious, clinging roots (e.g., ivy, Hedera species), with twining petioles (e.g., Clematis species), or using tendrils, which can be specialized shoots (Vitaceae), leaves (Bignoniaceae), or even inflorescences (Passiflora). Species of Parthenocissus (Vitaceae) produce twining tendrils that are modified stems, but which also produce adhesive pads at the end that attach themselves quite strongly to the support.
One odd group of vining plants is the fern genus Lygodium, called climbing ferns. Here, the plant's stem does not climb, but rather the fronds (leaves) do. The fronds unroll from the tip, and theoretically never stop growing. In the meantime, they can form thickets as they unroll over other plants, rockfaces, and fences.
Climbing plants as Garden plants
Gardeners can use the tendency of climbing plants to grow quickly. If a plant display is wanted fast a climber can achieve this. Climbers can be trained over walls, pergolas, fences etc. Climbers can be grown up other plants to provide additional attraction. Artificial support can also be provided. Some climbers climb by themselves; others need work, such as tying them in and training them.