Easy, cost effective, identification.
1D, linear barcodes
The most common barcode format scannable by the largest range of hardware. Although there are approximately lost of 1D barcode types, the majority in use are UPC, Code 128, Code 39, Interleaved 2 of 5 or EAN although others are becoming more popular. 1D barcodes store small amounts of information so are best for storing an "ID" that can be looked up in a database.
2D, stacked barcodes
Including PDF417, these are 1D barcodes stacked on top of each other. Scan using a barcode imager or by dragging a laser barcode scanner and down the barcode. Stacked barcodes can store a lot more information in them and are most commonly seen in the parcel delivery market.
2D, matrix barcodes
Matrix barcodes, such as QR Code or Data matrix or Aztec store even more information and can be scanned reliably by any camera or 2D barcode reader with the right decoding software. 2D barcodes are becoming more and more common and are starting to replace 1D barcodes as the technology becomes more prevalent.
Postal barcodes
Now on every letter posted, postal barcodes are primarily used by postal sorting offices but can now be scanned by most devices with a 2D barcode scanner. In the UK we use BPO 4 State code.