I'll try and explain this...
Bar codes essentially translate to keyboard input. That is from your application's point of view, it would look like someone has entered some data via a keyboard.
Now in order to actually "read" the barcode, you typically need a barcode scanner. Of course these scanners work specifically with specific types of bar codes (1D and 2D bar codes). However, what you have on a cell phone is a camera and not a barcode scanner. So basically what you'll need is software that does the following
- Takes picture of a barcode (that is the barcode is there in the picture somewhere)
- Processes this image to "extract" the barcode information from it
- Passes on this information to your application as if it were key strokes on a keyboard.
Nowadays you also have the concept of JPEG color barcodes. These are barcodes that look like colorful images.
http://www.sersc.org/journals/IJSIP/vol2_no4/4.pdf
multi-format 1D/2D barcode image processing library for cell phones
http://code.google.com/p/zxing/