"Card reader" is the generic term for an input device that reads flash memory cards. It can be a standalone device that connects to a computer via USB or it may be integrated into a computer, printer, or multifunction device. In fact, most multifunction printer/scanner/copiers now have built-in card readers.
Most card readers accept multiple memory card formats, including compact flash (CF), secure digital (SD), and Sony's Memory Stick. Some card readers accept various other formats such as XD, SmartMedia, Microdrive, and Memory Stick Pro Duo cards.
The purpose of a card reader is, not surprisingly, to read the data from a memory card. When you place a memory card into a card reader, it will often show up on your computer as a mounted disk. You can then view the contents of the memory card by double-clicking the card's icon. This icon typically appears on the desktop of Macintosh computers or inside "My Computer" on Windows machines.
Since memory cards most often contain pictures from digital cameras, a photo organization program may automatically open when you insert a memory card into you card reader. This provides an easy way of importing your pictures into your photo album. If you don't want to import photos using the program, you can simply close the program and the card will still be mounted on your computer.
Once you decide to remove the card, make sure you unmount or "eject" the disk before physically removing the card. This will help prevent the data on the card from becoming corrupted.