Terahertz (THz) wave is also known as T-ray.It′s wavelength ranging from 30 μm to 3000 μm,between microwave and infrared.This article describes the characteristics and applications of terahertz waves.Some common sources are also described in detail.Calculating Equations for the key parameters are given.The atmosphere attention is analyzed,which is the main factor that limits the application of THz.As the technology developes,THz wave will benefit people a lot,just like optical and microwave.