What is Humic Acid?
Humic acids - the fraction of humic substances that is not soluble in water under acidic conditions (pH < 2) but is soluble at higher pH values. They can be extracted from soil by various reagents and which is insoluble in dilute acid. Humic acids are the major extractable component of soil humic substances. They are dark brown to black in color.
Fulvic acids - the fraction of humic substances that is soluble in water under all pH conditions. They remain in solution after removal of humic acid by acidification. Fulvic acids are light yellow to yellow-brown in color.
Humin - the fraction of humic substances that is not soluble in water at any pH value and in alkali. Humins are black in color.
With the following article you would know humic acid better.

About Humic Substances
(From http://www.hagroup.neu.edu/aboutha.htm)
Humic substances are ubiquitous in the environment. Their importance in agriculture and soil sciences has been acknowledged for over 150 years. Aquatic scientists have been slower in appreciating their importance, but now realize that they may constitute as much as 95% of the total dissolved organic matter in aquatic systems and often are equal to or greater than the concentrations of inorganic ions present. In many cases they act as the major buffering system, which has serious implications for acidification of lakes and rivers.

While important for microbial processes that drive many ecosystems in our world, the true interest to the chemist is their interactions with other elements and compounds. Humic substances have been documented to interact in some manner with over 50 elements from the periodic table.

These include nutrients, toxic metals, radionuclides (including the transuranium series) and the halogens. The latter can interact with humic substances in drinking water treatment to produce halogenated carcinogens such as chloroform and bromoform, which are then directly introduced into the public drinking water with obvious health consequences. Toxic metals and micronutrients can be made either more available to organisms or actually sequestered so as to reduce their toxicity or beneficial value. Furthermore, humic substances contain long-lived (almost stable populations) of free radicals, which are capable of reducing inorganic species such as Hg, Cr, and Pu to name a few. They are also capable of interacting with anthropogenic organic compounds such as polycyclic aromatic hydrocarbons, including the known carcinogen benzo(a)pyrene, again altering the chemical reactivity of these important chemicals.

The mechanisms of many of these interactions are unclear. That is a result of our lack of knowledge of the structural components of humic substances. While it is true that we understand certain gross structural characteristics, i.e. many toxic metals are believed to be complexed through carboxyl groups but Cu appears to be bound through some unidentified nitrogen moities, and the stabilization of free radicals appears to be through quinine / semi-quinone structures, we have no structural knowledge that allows us to predict the extent of these reactions under given physico-chemical regimes. Furthermore, without structural knowledge, measurement and prediction of kinetics of reactions is impossible.

A further riddle concerns the nature of organic compounds produced by biological and geochemical processes that contain structures that can complex metals, sequester anthropogenic organic compounds, oxidize and reduce elements to and from toxic forms, photosensitize chemical reactions, and enhance or retard the uptake of toxic compounds or micronutrients to plant and microbial organisms? Without structural knowledge of humic substances, we will continue to rattle around in the black box of ignorance when asked to predict and forecast the impacts of chemical and biological actions on our environment. Those of us who have studied these compounds know their importance to chemical (another facet of humic substances is their interference in industrial processes such as Al processing), agricultural, environmental and even health issues (humic substances are widely used in the treatment of many animal maladies and their potential for use in human health is being explored).

While the study and eventual elucidation of the structure of these complex mixtures might at first glance seem esoteric, they are important components in processes that touch an extremely broad suite of scientific disciplines.

What this means for your plants:

• Humates are bio-stimulants - almost anything that grows will benefit from application of humates.
• Humates stimulate seed germination and viability, and root respiration, formation and growth.
• Humates produce thicker, greener, and healthier foliage.
• Humates produce more, larger, longer lasting, and more beautiful flowers.
• Humates increase the protein, vitamin, and mineral contents of most fruits and vegetables.
• Humates supply growing plants with food. They also act in other important ways to make soil more fertile and productive.
• Humates help retain water-soluble inorganic fertilizers in sandy soils releasing them as needed to the growing plants.
• Humates increase the water holding capacity of sand helping plants to resist droughts and reduced water conditions.
• Humates reduce fertilizer requirements and increase yields in most crops.
  
• Humates increase aeration of the soil.