Ionic liquids (ILs), which consist solely of ions, have turned out to be a special category of solvents with remarkable physicochemical characteristics and versatile utility in chemistry and materials science. Their structure and characteristics are thoroughly covered in this chapter, which also establishes the basis for their classification into a number of functional types, including task-specific, chiral, tunable polarity solvent, protic, basic, neutral, bio-ILs, poly(ILs), and supported ILs. Every class is made to perform particular functions in industrial, biological, and chemical processes. Modern characterization techniques like FTIR, XPS, NMR, TGA, and TEM are discussed, along with the synthesis of ILs. Viscosity, surface tension, ionic conductivity, solubility, polarity, thermal stability, and low vapor pressure are just a few of the precise physicochemical characteristics that make them ideal for use in green chemistry. In many fields of research and industry, ILs have evolved as solvents, catalysts, electrolytes, and functional materials due to their ability to control precisely these characteristics through cation and anion modifications.