We provide an overview of the design and capabilities of the near-infrared spectrograph (NIRSpec) onboard the James Webb Space Telescope. NIRSpec is designed to be capable of carrying out low-resolution ($R=30-330$) prism spectroscopy over the wavelength range $0.6-5.3 \mu$m and higher resolution ($R=500-1340$ or $R=1320-3600$) grating spectroscopy over $0.7-5.2 \mu$m, both in single-object mode employing any one of five fixed slits, or a 3.1$\times$3.2arcsec$^2$ integral field unit, or in multiobject mode employing a novel programmable micro-shutter device covering a 3.6$\times$3.4 arcmin$^2$ field of view. The all-reflective optical chain of NIRSpec and the performance of its different components are described, and some of the trade-offs made in designing the instrument are touched upon. The faint-end spectrophotometric sensitivity expected of NIRSpec, as well as its dependency on the energetic particle environment that its two detector arrays are likely to be subjected to in orbit are also discussed.