Experience is a feature added for kerbals as of version 0.90.0 'Beta'.  Kerbal pilots, engineers and scientists gain experience for completing a variety of achievements. Experience gives them additional abilities, making crew members grow more useful over time.
Planting a flag on other planets and moons grants different amounts of experience depending on which one the flag is planted on. The experience is gained only once, so planting a second flag on the same planet will not provide additional experience.
|Planet/Moon||Flag Planting XP|