- Increased Appeal to nearby tiles
- Units cannot embark from or disembark to their tile
Cliffs increase the Appeal of the tile and present a defense against naval invasion. They usually come in "chains" - that is, in a series of nearby tiles - and you can use that fact to found a city which will be (almost) completely safe from naval invasions, since enemy land units will have great difficulty reaching it from the sea! Later, you could use the increased Appeal of Cliffs to make Seaside Resorts in nearby lowland tiles.
Note that a Harbor may still be built next to a cliff. When your units use it, they will be able to pass the Cliffs to Embark or Disembark. Enemy units won't, however.