This wiki is about 0.16, the current stable version of Factorio.
Information about 0.17, the current experimental version of Factorio, can be found on wiki.factorio.com.
Cliffs are part of the natural terrain generation. Cliffs are usually generated in lines with gaps, creating significant obstacles that have to be avoided until they can be blown up using Cliff explosives. They do not drop any resources when removed. Driving into cliffs does not damage the vehicle.
Cliffs should not be generated in the starting area, but due to a bug, they can be generated there. While it looks like they create elevation, this is just a visual trick and no elevation is actually present. This means that other entites can be built around cliffs as usual, including underground belts/pipes going "under" cliffs.
Turrets can be placed "atop" cliffs for a tactical advantage - firing from an area the enemy can't easily get to. Players should be wary of the environment, as it's possible to be cornered in an area surrounded by cliffs; however, players can also use such a setup to funnel enemies into a specific area. Cliff walls going straight across and facing north can have long handed inserters move items past them, though cliffs of any other nature are too thick for such inserters to reach across.