A personal best, or PB, also called a personal record or PR in some regions, is the fastest time a runner has ever recorded for a given race distance. Runners track PBs separately for each distance, like 5K, 10K, half marathon, and marathon, since times at different distances aren't directly comparable.
They mean exactly the same thing, your fastest recorded time for a distance. PB is more common in India, the UK, and Australia, while PR is the more common term in the United States.
Consistent training, a well-paced race strategy like a negative split, and picking a flat, well-organised course all help. Tapering properly before the race also matters more than most runners expect.
Most runners only count times from official, measured races with accurate timing, since course distance and conditions can vary a lot elsewhere. A fast training run on an uncertified route usually isn't treated as a true PB.
Because performance at one distance doesn't translate directly to another, a strong 10K time doesn't predict an exact marathon time. Each distance has its own pacing and endurance demands, so each gets its own personal best.