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The Integrated Science Instrument Module (ISIM) includes Webb's science instruments. The OTE is the eye of the Observatory and it gathers the light coming from space and provides it to the science instruments located in the ISIM. The backplane is like the Webb's "spine". It supports the mirrors.

The sunshield subsystem separates the observatory into a warm sun-facing side (spacecraft bus) and a cold anti-sun side (OTE and ISIM). The sunshield keeps the heat of the Sun, Earth, and spacecraft bus electronics away from the OTE and ISIM so that these pieces of the Observatory can be kept very cold (The operating temperature has to be kept under 50 kelvins or -370 deg F).

The Spacecraft Bus provides the support functions for the operation of the Observatory. The bus houses the six major subsystems needed to operate the spacecraft: the Electrical Power Subsystem, the Attitude Control Subsystem, the Communication Subsystem, the Command and Data Handling Subsystem, the Propulsion Subsystem, and the Thermal Control Subsystem.

The momentum flap balances the solar pressure on the sunshield, like a trim flap in sailing. It's not adjustable on orbit, but it is while it's on the ground.

More Information About the Observatory Elements:

Integrated Science Instrument Module (ISIM)
Optical Telescope Element (OTE), mirrors, and backplane
Spacecraft Element (Spacecraft Bus and Sunshield)
The ISIM includes the following instruments:

Near-Infrared Camera, or NIRCam, - provided by the University of Arizona
Near-Infrared Spectrograph, or NIRSpec - provided by ESA, with components provided by NASA/GSFC.
Mid-Infrared Instrument, or MIRI - provided by the European Consortium with the European Space Agency (ESA), and by the NASA Jet Propulsion Laboratory (JPL)
Fine Guidance Sensor/Near InfraRed Imager and Slitless Spectrograph , or FGS/NIRISS- provided by the Canadian Space Agency