A gibberellin is a major type of plant hormone (auxins are the other major type). Gibberellins are produced in comparatively large quantities by plant seeds, algae, mosses, ferns and fungi (especially fungi in the genus Gibberella). Gibberelic acid is commonly used in seed germination experiments to induce growth.
Gibberellins can produce dramatic growth, especially in plant stems. If a plant under-produces gibberellins, it becomes a dwarf. Gibberellins are relatively expensive, but have been used in the production of seedless grapes and navel oranges and have been found to reverse the effects of some herbicides.
This writeup is based partly on work I did for the science dictionary at http://biotech.icmb.utexas.edu/