I think the easiest way to determine the difference between an input and output device, and subsequently answer your question is to think of what direction information is going with relation to the device you are using.
For example, if you are using a PC mouse, the movements of your hand/fingers is being changed into electrical impulses to tell the computer what to do. This, therefore, is an input device. This means you are sending information TO the computer.
Each time you move the mouse or click on an icon, the computer sends you some feedback with the speaker or by what the screen displays (cursor moves, icon changes color, etc.). This tells you that the monitor is an output device since it is providing some sort of output to you, the user.
After you have the concepts above understood, you can carry it over into other devices such as flash drives, hard drives, CD-read drives, CD read/write drives, printers, etc.
For example, a printer is an output device because you are receiving output from your computer.
A read/write CD or DVD drive is both an input and output device because not only can you get information into your computer FROM it, you can also send information TO it from your computer.
I hope this helps.