If you drag and drop while holding the command button, it will move the file instead of copying it.
If you drag and drop a file between two locations on the same filesystem, it will move the file instead of copying it. Hold the command button to copy.
Look for the green + in a circle when you're ready to "drop." If that's present, you're copying it (the copy is like "adding" the file to an additional spot, hence the plus sign). If it isn't, you're moving it.