DRS (Decision Review System) is a technical process which helps to find accurate results in making decisions for match officials & this is implemented in order to fix the umpiring errors & also DRS helps the game to be more transparent.
Now coming to DRS, each side is given two unsuccessful review requests per innings in a test match & when it comes to ODI format one unsuccessful review per innings.
Fielding team can request for DRS if the umpire gives ‘not out’ decision & batting team can request DRS if umpire gives ‘out’ decision, the DRS review request must be made by Captain if fielding team want to go for DRS & if it is batting team batsman who faced the ball must take the review. And there will be only 15 seconds time for anyone who want to request DRS, after 15 seconds umpire won’t accept the review. And in case if the review turns out to be successful(wrong decision by on-field umpire), then the unsuccessful review will be carry forwarded for whoever gets the review successful.