A scorer in the sport of cricket is someone appointed to record all runs scored, all wickets taken and, where appropriate, number of overs bowled. In professional games, two scorers are appointed. This allows them to frequently check that their records agree with each other's as well as with the umpires'.
The scorers have no discretion as to whether runs are scored, wickets taken or overs bowled as they must accept all instructions and signals given to them by the umpires. So that the umpire knows that they have received a signal, they are required to immediately signal back to the umpire to acknowledge it.
Generally, and always in the professional game, the scorers use specific cricket scoring books. Coming in many different styles, they will normally allow the scorer to break down a bowler's analysis, an individual batsman's score, and sometimes the number of balls he faced, as well as noting down team scores.
The scorers have a clearly defined role under the Laws of Cricket, and a modern scorer's role is both important and complicated. This is in stark contrast to an earlier time, when a batsman's runs were carved on a stick - which is the root of the slang term "notches", meaning runs.