The James Webb Space Telescope (JWST) is a key element in NASA's Origins program, which has the goal of understanding the formation of galaxies, stars, planets and ultimately, life. JWST is specifically designed for discovering and understanding the formation of the first stars and galaxies, measuring the geometry of the Universe and the distribution of dark matter, investigating the evolution of galaxies and the production of elements by stars, and the process of star and planet formation. This is a document from JWST project website at NASA/Goddard describing the basic science objectives of JWST (pdf).
The NGST Ad Hoc Science Working Group created in 1999 the Design Reference Mission (DRM), a set of hypothetical observing programs identifying a core science program for the JWST. Associated with these observing programs, a suite of potential astronomical targets were identified, each with their expected physical properties (number density and brightness) and desired observation modes (wavelength band, spectral resolution, number of revisits). Using the JWST Mission Simulator (JMS) each possible JWST design is tested for accomplishing the most number of DRM goals within the allotted time and budget.