This page covers the Column Tool.
There are only a handful of columns in the base game. But with Create-A-Style, there are still many possibilities. See the recolors behind the originals in the picture for some ideas.
Columns create an area of flooring potential above them. You can achieve a "carport" look with columns and a floor.
You can use columns to create a floor above them and then delete the columns to achieve an off-the-ground look for your home. It causes the room to appear to "float". Create columns and then put a floor on top and then delete the columns. The floor remains. You can build a room on the floor. Of course if it is to be used, there has to be stairs or something to enable your Sims to reach the room.
To get a good idea of some of the column possibilities, look at some of the pre-made lots. To check out the available columns, see the Columns section of the Object Catalog.