It is a Russian designed variant of the T-72, with a less powerful engine than the T-80.
"Initially seen as an entirely new design, the production model is in fact based on the T-72BM, with some added features from the T-80 series. The T-90 features a new generation of armor on its hull and turret. Two variants, the T-90S and T-90E, have been identified as possible export models."
http://www.fas.org/man/dod-101/sys/land/row/t-90.htm
They are assembled locally from Russian kits, after India originally purchased a number of pre-built models.
"India on Wednesday rolled out the first locally-assembled T-90 as part of a deal with its main arms supplier Russia to put together as many as 186 such frontline battle tanks in the next few years, officials here said.
"Russia has already supplied 124 off-the-shelf T-90s worth 650 million dollars after signing the mammoth military deal in 2001 to help India to upgrade its ageing T-72 tanks, which has scant protection against nuclear attacks.
http://pakistantimes.net/2004/01/08/top10.htm