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 grants additional abilities, making crew members grow more useful over time.
Orbiting, planting flags, and escaping from the gravity of planets and moons grants various amounts of experience depending on the planet. Each Kerbal can only gain the experience once per planet, so planting multiple flags or repeatedly orbiting a planet do not add provide any further experience for the same Kerbal. Additionally, experience from a given planet or moon is only granted for the most valuable achievement, so orbiting the mun (3 XP) and planting a flag on it (5 XP) will only grant 5 XP, not 8.
|Planet/Moon||Flyby/Escape (XP)||Orbiting (XP)||Landing (XP)||Flag planting (XP)|