An adjuvant is any chemical that is added to an herbicide in order to increase its effectiveness as a weed killer. There are many different adjuvants that are either added by the herbicide's manufacturer or by the gardener applying the herbicide; each has its own specific effect on the herbicide. Surfactants are one example of this class of chemical, and understanding how they are specifically used in herbicides is key to using them properly.
To understand how surfactants solve the problem of making herbicides stick to weeds, it is necessary to first examine how the "stickiness" of herbicides is a problem. Foliar spray herbicides that kill weeds and unwanted grasses make direct contact with the unwanted vegetation and only become lethal to unwanted vegetation after the plant absorbs and metabolizes the active herbicide ingredient — these types of herbicides are called "postemergent" formulations. Therefore, if you spray a contact herbicide on unwanted vegetation and the herbicide is washed away by rainfall, blown about by wind or otherwise disturbed, the herbicide will be ineffective at best, as it's been removed before the unwanted vegetation fully metabolizes the lethal chemical.
A surfactant, in the most general sense, is any chemical designed to change the surface tension of a surface. Surfactants are commonly used in many household products, including liquid dish soap. In the context of herbicides, surfactants increase an herbicide's ability to penetrate the internal vasculature of the target vegetation, rather than the herbicide's beading up on the surface of the leaf, where it can be easily removed by the aforementioned natural factors. Surfactants are sometimes added to herbicides by the manufacturer. Alternatively, you can purchase an herbicide surfactant separately at gardening supply stores and mix it with the herbicide yourself prior to usage.
There are many different types of surfactants sold for use with herbicides. Some common ingredients of these surfactants include seed oil, silicone, vegetable oil, nonionic organo-silicone and other chemical compounds. Other surfactants include specific types of nonionic, anionic, cationic compounds and organo-silicones. For some herbicides, adding a surfactant is a matter of personal preference. For others, mixing in a surfactant prior to use is considered essential to the herbicide's functioning properly.
You should always follow the mixing instructions on any surfactant you purchase, and make sure that the surfactant is mixed and used in a manner that is compliant with the manufacturer's instructions on both the herbicide and the surfactant. Commonly, surfactants are mixed into herbicides at a ratio of one-half to two parts per acre, or 0.25 percent of total herbicide volume. Due to their potential for health and safety hazards, surfactants should always be used in accordance with the manufacturer's instructions.