When managing files in OS X, common practices are to drag and drop them in various locations to move them or copy them between storage devices. Simple tasks include dragging individual items between folders, or even selecting multiple items to move at once for easy consolidation of files. Apple's file-system browser, Finder, is fairly straightforward in this respect, but in addition to these basic routines are some hidden options you can use to enhance the basic drag-and-drop features.
1. Copy files
When you drag a file to a new location on the same disk, the default behavior is to move the files to that location. Sometimes odd permissions settings only allow you to copy from one area to another, but provided you have full access to the source file and destination location, then dragging should just move the files. However, if you hold the Option key down when dragging one or more files, then the system will copy the files instead of moving them, even if they are moved within the same folder.
You can use this not only to copy files, but because it acts on files in the same folder you can also to quickly duplicate files. To do this, just hold the option key and drag a file or selection of files until you see the green plus sign appear, and then release the files and the system will make duplicates (granted just pressing Command-D on the keyboard will do the same thing, but this is another available method as well).
2. Make aliases
In addition to copying files, you can create links to them. OS X supports both unix links and aliases (Mac-specific links), which are in essence directory entries that redirect you to a file in another location. This allows you to access the same file in its original file path from multiple places without duplicating it.
To make an alias, just as you would copy a file or selection of files, you can drag them and then press the Command key in addition to the Option key. This time instead of seeing a plus sign next to the pointer you will see a curved arrow. When you release the files then the system will create aliases to them. This can be convenient if, for instance, you want to create a folder of links to your commonly used applications, to then put in the Dock as a stack for easy access.
3. Navigate through folders
The last option when dragging files is to navigate the file system. In the past you would have to reveal both your source file as well as your desired destination for it to copy or move that file; however, in OS X you can navigate through the file system by hovering your dragged file selection over a folder until it highlights. Then you can wait for a brief delay before the folder opens, or you can press the space bar to open the folder immediately (this is particularly easy to do in Column view).
Undoing and preventing changes
A final note to keep in mind when dragging and dropping files is how to undo any changes or stop an act in progress. One unfortunate side effect of dragging and dropping is that you may inadvertently place files in the wrong locations. To undo any changes you made you can easily press Command-Z (the universal "undo" command), and the Finder should revert them.
In addition to undoing changes, you can stop a dragging event in its tracks. Sometimes when dragging and dropping you may realize you've selected too many files or have missed one file. In these instances you can release your Option or Command keys and then place the dragged files back into their source folders, or you can just hit the Escape key to cancel the drag event, and any selected files will swoop back to their original locations.