The ASTROSAT is India's first observation satellite launched into orbit, to study distant celestial bodies. The reason you've been hearing about it so much is because it's India's first satellite of its kind, built at a cost of Rs 178 crore (excluding the cost of launching the satellite) and we're also ahead of a lot of other countries. So far, only USA, Japan, and a few countries in the EU have observatories in space.

 It's a satellite that we will use to remotely study celestial bodies and the behaviour of galaxies using radiation analysis. For the purpose, the ASTROSAT has 5 instruments on board: 

• UVIT : Ultra-Violet Imaging Telescope
• LAXPC : Large Area X-rat Proportional Counter
• SXT : Soft X-ray Telescope
• CZTI : Cadmium-Zinc-Telluride Imager
• SSM : Scanning Sky Monitor

The first four instruments are what hep use observe a celestial body or phenomenon by recording the various types of radiation it gives off; from the visible spectrum, to infrared and ultraviolet radiation, and hard and soft x-rays. Therefore, these four are aligned to look at the same point in space at one time. The fifth instrument, the SSM, is in fact three small position sensitive x-ray detectors that continuously patrol the sky, looking for changes in the radiation from celestial objects, so that the the satellite can be rotated to point the other four instruments there. Think of the SSM as the scout for the satellite. Of course, this entire setup has to be in space because our atmosphere is largely opaque to ultraviolet radiation, x-rays, gamma rays and also parts of infrared and microwave radiation.

ASTROSAT will be studying various celestial bodies, as well as gathering data to hep us piece together how stars and galaxies are born. The satellite will undergo extensive tests for the first six months after launch. The next six will be devoted to observations and data collection predetermined by the mission team. But a year after launch, ASTROSAT's services will be available to any scientist in India, with a good proposal, reviewed by ISRO.
Once it passes the two-year mark, international scientists will be allowed to apply as well. Once a significant amount of time has passed for each bit of observation, the data will be made available to the public.