Abscisic acid is a single compound unlike the auxins, gibberellins, and cytokinins. It was called "abscisin II" originally because it was thought to play a major role in abscission of fruits. Though ABA generally is thought to play mostly inhibitory roles, it has many promoting functions as well.
In 1963, abscisic acid was first identified and characterized by Frederick Addicott and his associates. They were studying compounds responsible for the abscission of fruits (cotton). Two compounds were isolated and called abscisin I and abscisin II. Abscisin II is presently called abscisic acid.
ABA is a naturally occurring compound in plants. It is a sesquiterpenoid (15-carbon) which is partially produced via a certain pathway (mevalonic pathway) in chloroplasts and other plastids.
Because it is sythesized partially in the chloroplasts, it makes sense that biosynthesis primarily occurs in the leaves. The production of ABA is accentuated by stresses such as water loss and freezing temperatures.
It is believed that biosynthesis occurs indirectly through the production of carotenoids. Carotenoids are pigments produced by the chloroplast which have 40 carbons. Breakdown of these carotenoids occurs in a complex mechanism which produces ABA.
The transport of ABA can occur in both xylem and phloem tissues. It can also be translocated through paranchyma cells. The movement of abscisic acid in plants does not exhibit polarity like auxins. ABA is capable of moving both up and down the stem.
The various roles of ABA are
Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis). Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots. Induces seeds to synthesize storage proteins.
Has some effect on induction and maintanance of dormancy.
(This information taken from http://www.plant-hormones.info/abscisicacid.htm)