Initially, the PropertyGrid is in categorized mode. This is the mode where you can build its content. It displays the properties hierarchized in a tree and each level can be collapsed if needed. The depth (number of levels) of the tree has no limit.

On the image above, you can see two levels of properties, most of them being embedded in a subcategory. Notice how the indentation makes things easy to read. This is not like some competing implementations that display the first subcategory at the same level as a root category.

On this second image, the display has been switched to non sorted flat mode. It's like you were just removing all categories. Although this is not visible in this screenshot, you will maybe still see some +/- signs but they belong to grouped properties which can't be flattened.

At last, in sorted flat mode, categories are also removed but the properties are displayed in alphabetical order. This can be a good idea to let the user switch to this mode with the toolbar when the PropertyGrid contains a long list of non duplicate labels. It allows a fast retrieval of a given item known by name.
In flat mode, the left thin column can be hidden. This may be interessant to mimic the look of a list control.