Baserunners stand on or close to first base, second base and third base at the time a pitch is thrown. Once the pitch is thrown, baserunners can try to advance to the next base -- on a stolen-base attempt or after the ball is put into play. The ultimate goal of a baserunner is to score.
A baserunner can reach base safely after drawing a walk or recording a hit. Once on the bases, he attempts to advance base-to-base before reaching home plate without being tagged or forced out. In no circumstances can a baserunner skip a base.
A runner can also reach base after recording an out during his time at the plate. Such happens most frequently when a batter hits into a "fielder's choice," which occur when a fielder -- usually an infielder -- opts to field a batted ball and retire a runner at second, third or home opposed to throwing to or tagging first base to retire the batter-runner.
Since fielders have a right to occupy any space needed to catch or field a batted or thrown ball, runners are not given the "right of way" on the bases. If a runner is adjudged to have interfered with a defensive team's efforts, then the runner, the batter -- or both the runner and batter -- can be declared out for interference.
In such cases, the ball will be declared dead and all runners must return to their last legally occupied base at the time of the interference.
On the other hand, baserunners have the right to advance on the bases without being obstructed by the fielding team (except in cases when the fielder closest to the runner is in the process of making a defensive play). If a baserunner is obstructed, he is entitled to whatever base the umpire adjudges he would have reached if not for the obstruction.