Regulatory small RNAs, which range in size from 20 to 24 nucleotides, are ubiquitous components of endogenous plant transcriptomes, as well as common responses to exogenous viral infections and introduced double-stranded RNA (dsRNA). Endogenous small RNAs derive from the processing of helical RNA precursors and can be categorized into several groups based on differences in biogenesis and function. A major distinction can be observed between small RNAs derived from single-stranded precursors with a hairpin structure [referred to here as hairpin RNAs (hpRNAs)] and those derived from dsRNA precursors [small interfering RNAs (siRNAs)]. hpRNAs in plants can be divided into two secondary groups: microRNAs and those that are not microRNAs. The currently known siRNAs fall mostly into one of three secondary groups: heterochromatic siRNAs, secondary siRNAs, and natural antisense transcript siRNAs. Tertiary subdivisions can be identified within many of the secondary classifications as well. Comparisons between the different classes of plant small RNAs help to illuminate key goals for future research.