The standard league game is 501, double out: each player starts on 501, subtracts every throw, and must finish exactly on zero with a double (or the bullseye). One game is a leg, and a match is best of a number of legs — or, at higher levels, sets of legs.
Standings rank on match wins (often as points, such as 2 for a win), with legs won and lost as the first tiebreaker. Team leagues play several singles and pairs games per fixture and count games won.
The stat every darts player knows is the three-dart average: total points scored ÷ darts thrown × 3. It measures scoring power independent of results, and many leagues track it beside 180s and high checkouts.