Solid: particles close together, regular arrangement, vibrate about fixed positions. Fixed shape and volume.
Liquid: particles close but irregular, can slide past each other. Fixed volume, takes shape of container.
Gas: particles far apart, move quickly and randomly. No fixed shape or volume, fills the container.
Changes of state: melting (solid→liquid), boiling/evaporation (liquid→gas), condensation (gas→liquid), freezing (liquid→solid), sublimation (solid→gas), deposition (gas→solid).