Minecraft wiki entry for Ticking_area, associated text: Water flows and breaks non-solid blocks.
Lava flows, sets fire to nearby flammable blocks, destroys item entities, and interacts with water.
Fire spreads to adjacent flammable blocks.
Dropped items fall and despawn after 5 minutes if not picked up or collected by a hopper.
Passive mobs grow up, and move around normally.
Sheep eat grass and grow wool.
Sand, gravel, and concrete powder fall if the block beneath them is moved or broken.
Primed TNT falls and explodes.
Minecarts function normally.
Redstone mechanisms and devices function normally, provided they are entirely contained within ticking areas.
Crops, mushrooms, saplings, and nether wart grow to maturity if the circumstances are suitable.
Farmland changes its hydration in response to available water.
Leaves decay if not connected to a log.
Grass and mycelium spread to adjacent blocks.
Ice forms and melts in response to changes in lighting.
Rain, snow, and lightning occur and have their usual effects.
Hostile mobs move around as usual.