As the BBC news web site pointed out, last week marked two years until the launch of the James Webb Space Telescope (JWST). This is the replacement for Hubble, and is a much more powerful/sensitive device. The mirrors and optical components are now assembled and ready to be tested. It's a reflector. Radiation hits the main mirror, 6.5 metres across, is then reflected and focussed onto a much smaller secondary mirror in front of the main one and then reflected onto sensors at the centre of the main mirror. You can see this in the photo above.
The JWST will be sensitive to wavelengths long enough to see back to the early days of the universe. It's basic doppler effect: as the sources of light move away from us at increasing speed, the light we see from them lengthens in wavelength towards red and beyond (hence red-shift). Because the universe is expanding, the further away from us an object is, the faster it is moving away from us. [Good pub question: where is the centre of the observable universe? Answer: where you are.]
This is one of its sensors, for NIRCam - 2048 by 2048 pixels for near infrared wavelengths between 0.6 and 5 microns (600 and 5000 µm).
NIRCam is one of four instruments: NIRCam, NIRSpec, NIRSS and MIRI. MIRI images wavelengths between 5 and 28.5 microns with a resolution of 1024 by 1024 pixels. This is a gross oversimplification, and sections 20 to 23 of Nasa'a scientific FAQ give much more information about the cameras.
If you're interested in more fine detail about the NIR system, then try this NIRCam Instrument Overview paper from the University of Arizona. Nasa has a set of web pages, Explore the James Webb Space Telescope, with copious resources.
[Photos courtesy of Nasa]