There are all types of terrain in Pikmin and Pikmin 2. They don't really affect the game play, but they do change the atmosphere and the different types of enemies you may encounter. For example, electricity is far more common in indoor-like terrain. In Pikmin 2, man-made terrain, such as metal or tile, is more common than in Pikmin 1.
Pikmin 1
Most areas in the first Pikmin game have natural surfaces such as sand, soil and wood.
Grass
Found in most areas, except for the Forest Navel (Because it is underground).
Soil
This is what you and your pikmin will be walking on for a majority of the time. Soil is found in every area in the game.
Sand
Usually, when there is water, there is sand. Especially in The Distant Spring. In the Distant Spring, about half the amount of land is submerged in water, and that amount of land is sand. It can also be found in the small pond area in the Impact Site and the large lake in the Forest Navel.
Wood
Various tree stumps can be found in the game. They can be found in the Impact Site, Forest of Hope and the Distant Spring.
Cardboard
The Cardboard boxes in the Impact Site and the Final Trial can be walked upon by using a nearby rock as a platform.
Rusted metal
In The Final Trial, large slabs of rusted metal is used to cross the body of water separating the landing site, from the area where you battle, the Emperor Bulblax, which actually takes place in a large metal 'bowl' like arena.
Pikmin 2
In Pikmin 2, you will find a vast array of different terrain. Most are likely man-made, indicating that some caverns may have been once inhabited by people.
Natural
Snow
In some caverns, you may find the area to be covered in a blanket of snow. This terrain can be found in the Valley of Repose and its respective caverns.
Grass
A common terrain, mostly found in above ground areas. With the Awakening Wood being an common example. Grass-like terrain can be found in the Perplexing Pool, however considering the areas aquatic nature it's possible that it's actually moss.
Man-made
Concrete
Can be found in a few areas, most of the Valley of Repose is formed from concrete. This is due to it being believed to be a crosswalk of sorts.
Steel
Some caverns will feature steel-like terrain; commonly referred as the 'Battle field theme'. It features blue tinted metal, which is partially rusted. Objects such as poles and hatches can be seen. Oddly, by putting the camera at certain angles, one can see the whole area is being held by thick iron bars. The manhole in The Valley of Repose may also be made of steel.
Wood
In most toybox themed caverns the floor appears to be made out wooden floorboards. The final sublevel of Glutton's Kitchen features a red and white pic-nic blanket but it makes the same sound when walked on.
Carpet
Some caverns feature a slight variation of the toybox theme, as instead of wooden floorboards, it features a simple gray-white carpet. It is also seen in the 2-player mode exclusive, Carpet Plain.
Tile
This style consists of simple turquoise-coloured tile filled floors and walls, which have sand filled in between each tile. Areas with this style usually have water, which is typically found in small descents of ground. The tiles may be interrupted by sand or a metallic floor.
Man-made objects
Inside toybox themed caverns (Glutton's Kitchen for example), you can occasionally find toy train tracks, a xylophone, and a few plates (Sublevel 5 of Glutton's Kitchen only) you are able to travel on. Along with several outdoor themed caverns such as the Snagret Hole, you may find a useless shovel and pail. Though it is possible walk on, this is more or less pointless due to the fact that Pikmin can slide off the pail easily. Also, Paper Bags can be found in some areas as well.
Stone formations
Inside the Perplexing Pool, there are odd stones formations that are mysteriously shaped, some of which are walk-able. These odd formations do not appear to be natural, ranging from simple pathways to maze-like structures.
Scales
In several areas, there are sets of scales. They can be used to lift Pikmin and the Captains to higher areas, and several sets of these scales must be navigated to obtain treasure.
Crystals and Ice
In Pikmin 3, shards of glass and large crystal formations are present and can serve as obstacles or have valuable items encased inside of them. They can be shattered by being continuously hit with Rock Pikmin.