Nanoparticles and nanofillers have undergone substantial research and are known to have profound industrial applications due to their significant reduction in the cost of production and modification of properties. In this review, a thorough review of nanoparticles and nanofillers is performed. The different types of nanofillers are reviewed and compared, with the main classification used being inorganic nanoparticles (metals, oxides, carbon-based nanofillers) and organic nanoparticles. The most important examples of the use of these nanoparticles are tabulated according to their class (inorganic or organic). The primary synthesis strategies and characterization methods are also reviewed. Additionally, safety issues in manufacturing, applications, and handling are discussed.