Android programs can interact with eachother using intents. Intents are a little like remote procedure calls: you ask the other program for a certain action (e.g. scan a barcode) and the other program will perform this task for you. The result is returned when the task is complete.
If the user has installed the ZXing Barcode Scanner, you can just use an intent to scan a barcode. The Barcode Scanner will then start, let the user scan the code and return the result to you.
More information about this scanner can be found on the Google Code page of this project: http://code.google.com/p/zxing/wiki/ScanningViaIntent