Terrain

Difficult terrain is any terrain that impedes your movement, ranging from particularly rough or unstable surfaces to thick ground cover and countless other impediments. Moving into a square of difficult terrain (or moving 5 feet into or within an area of difficult terrain, if you're not using a grid) costs an extra 5 feet of movement. Moving into a square of greater difficult terrain instead costs 10 additional feet of movement. This additional cost isn't increased further when moving diagonally. You can't Step into difficult terrain.
Movement you make while jumping ignores the terrain you're jumping over. Some abilities (such as flight or being incorporeal) allow you to avoid the movement reduction from some types of difficult terrain.

Hazardous terrain damages you whenever you move through it. An industrial acid vat, a pit of molten slag, and a field of razor-sharp grass on a strange planet are all examples of hazardous terrain. The amount and type of damage depend on the specific hazardous terrain.

A narrow surface is so precariously thin that you need to Balance or risk falling. Even on a success, you're off-guard on a narrow surface. Each time you are hit by an attack or fail a save on a narrow surface, you must succeed at a Reflex save (with the same DC as the Acrobatics check to Balance) or fall.

Uneven ground is an area unsteady enough that you need to Balance or risk falling prone and possibly injuring yourself, depending on the specifics of the uneven ground. You're off-guard on uneven ground. Each time you are hit by an attack or fail a save on uneven ground, you must succeed at a Reflex save (with the same DC as the Acrobatics check to Balance) or fall prone.

An incline is an area so steep that you need to Climb using the Athletics skill in order to progress upward. You're off-guard when Climbing an incline.