Innings and outs
A baseball game is divided into nine innings, and each inning gives both teams a turn to bat. A turn ends when the fielding side records three outs. Nothing else ends it: there is no clock counting down, and a team that keeps avoiding outs keeps batting. That is the structural difference between baseball and almost every other major sport.
Because the game cannot end in a tie, extra innings are played until one team leads at the end of a complete inning. The home team bats second, which is why a home side leading after the visitors have batted in the ninth simply wins without taking its turn.
The absence of a clock also removes the concept of protecting a lead by holding the ball. A team ahead by one run in the ninth still has to record three outs, and no amount of tactical caution shortens the task.
The count
Every plate appearance is a contest between one pitcher and one batter, scored in balls and strikes. Four balls and the batter walks to first base for free. Three strikes and he is out. The running tally is called the count, and it shifts the advantage between the two players pitch by pitch.
A batter ahead in the count can afford to wait for a pitch he likes, which is why pitchers who fall behind get hit hard. A batter behind in the count has to protect the plate and swing at pitches he would otherwise leave, which is why the first strike of an at-bat matters far more than it appears to.
A ball hit outside the field of play counts as a strike unless the batter already has two, in which case the at-bat simply continues. This is the rule that allows a plate appearance to stretch to ten or twelve pitches and quietly wear a starting pitcher down.
Runners, scoring and the modern rules
A runner scores by touching all four bases in order. He can advance on a hit, on a walk that forces him along, or by stealing — running to the next base while the pitch is on its way, which is a calculated risk against the catcher's arm.
Fielders record outs by catching a ball before it lands, by tagging a runner who is not on a base, or by reaching a base with the ball before a runner who is forced to advance. Almost every defensive play in the sport is one of those three things.
The most significant recent changes addressed pace and action rather than the fundamentals. A pitch clock limits the time between pitches, restrictions on defensive positioning stopped teams stacking fielders on one side of the infield, and larger bases made stealing slightly more attractive. The rules of the contest are unchanged; the amount of standing around is not.