Classified by Use:

• Spray modifiers: A type of adjuvant used to alter the physical characteristics of the spray solution. For example, Stickers, Spreader-stickers, Foaming agents, Drift control agents.
• Utility Adjuvants: Widen the range of conditions under which a given formulation is useful. For example: Antifoam agents, Compatibility agents, Buffering agents.
• Activator Agents: Increase herbicide activity, may increase herbicide absorption, spread, provide rainfastness, and/or decrease phototransformation of the herbicide. For example, Surfactants, Crop oils, Crop oil concentrates Water Conditioners.

Classified by Charge:

Chemical charge classes of surface active agents (surfactants).The active moiety contains both hydrophilic and lipophilic segments.

• Anionic – negative charge
• Cationic - positive charge
• Nonionic (NIS) – no charge
• Ampholytic – both positive and negative charges

Classified by Chemicals:

• Non-inclusive listing of activator adjuvant chemical classes 
• Alkylaryl polyglycol ethers
• Alkyl phenol ethoxylates (APE) 
  nonylphenol ethoxylates (NPE)
• Linear alcohol ethoxylates
• Alkyl glycosides
• Cationic tallow amines
• Phosphate esters
• Organosilicones
• Fatty acids 
  - saponified 
  - esterified: methyl or ethyl

From: ADJUVANTS: Principles and Uses, written by Donald Penner from Michigan State University