Nanotechnology is a cutting-edge field of science, which allows the manipulation of materials having dimensions in nanometers. Nanotechnology has numerous applications in almost all the areas, including human health, environment, bio-diagnostics, drug delivery etc. Excessive and over-use of nanomaterials has led to the uncontrolled release of nanomaterials into the environments which are toxic to the living organisms. The present review has attempted to provide a detailed overview of the nanoparticles, their synthesis, classification, applications and toxic effects. Different methods of nanoparticles synthesis like physical, chemical and biological methods have been addressed. Physical and chemical methods for nanoparticles synthesis are less eco-friendly and contaminate the environment. Recently, biological methods have gained significant attention and are frequently used because they are eco-friendly and cost effective. Due to enormous properties of nanoparticles they are suitable for various commercial, domestic, medical and environmental applications exist in different forms. This paper also reviews exposures to toxic effects of nanoparticles on human and environment.