Nanotechnology has recently found applications in many fields such as consumer products, medicine and environment. Nanoparticles display unique properties and vary widely according to their dimensions, morphology, composition, agglomeration and uniformity states. Nanomaterials include carbon-based nanoparticles, metal-based nanoparticles, organic-based nanoparticles and composite-based nanoparticles. The increasing production and use of nanoparticles result in higher exposure to humans and the environment, thus raising issues of toxicity. Here we review the properties, applications and toxicity of metal and non-metal-based nanoparticles. Nanoparticles are likely to be accumulated in sensitive organs such as heart, liver, spleen, kidney and brain after inhalation, ingestion and skin contact. In vitro and in vivo studies indicate that exposure to nanoparticles could induce the production of reactive oxygen species (ROS), which is a predominant mechanism leading to toxicity. Excessive production of ROS causes oxidative stress, inflammation and subsequent damage to proteins, cell membranes and DNA. ROS production induced by nanoparticles is controlled by size, shape, surface, composition, solubility, aggregation and particle uptake. The toxicity of a metallic nanomaterial may differ depending on the oxidation state, ligands, solubility and morphology, and on environmental and health conditions.