The generic icons that Apple and third-party developers provide for files and folders are usually enough to help you identify file types in OS X, but there are times when you might want to use a different representation. One option that Apple offers is the ability to show file previews as icons, but this is only available for some file types and not for others.
An alternative option is to add custom icons to files and folders, which can be a fun and useful way to give your system a unique look. To do this, simply select an item and press Command-I to get information on it, and then with an image copied to the clipboard, you can select the icon at the top of the information window and press Command-V to paste and change it.
You can also drag a file type to the icon in the information window to use that file type's generic icon. For example, if you drag any PDF to the icon field of the information window of another file, then that file will look like a generic PDF icon.
By using Apple's inspector panel, you can easily customize multiple items at once. Unlike the standard information window, the inspector panel is a single hovering window that will give you information on the current selection, as opposed to a single file only. With this panel you can quickly apply changes to documents simply by selecting one or more of them instead of having to get information on each one.
First, open the inspector panel by hitting Option-Command-I, and then select a file of choice. You will see the panel's details change to reflect the file's data. To change the icon, as you would with the standard information window, simply copy an image and paste it to the icon field, or drag a file to adopt the generic icon for that file type.
Since the inspector is a persistent window, you can use it to apply changes without having to open a new information window each time.
Another approach to this is to use the inspector to apply icon changes to several items at once. To do this, select multiple items in the Finder, and the inspector will change to a multi-item information window. You can do this by holding the Shift or Command keys and then select by clicking, dragging, or using the arrow keys. You can also use the system's "Find" feature (press Command-F in the Finder) to isolate groups of items and then select them all within the Find window.
You can make your file selection either before or after opening the inspector window, but with the selection done you can similarly apply an icon change, and all selected items will be altered.
As with the standard info window, you can also undo these changes for multiple files by selecting the icon in the inspector window and pressing the Delete key to remove it. This should revert the icon back to its default view.