2D Barcode Symbologies are the next step in Symbologies from the known 1D Barcode Symbologies we know and love today. The main form we see is UPC like you would see in the grocery store. Manufacturing and other industries utilize mainly 3of9 and 128 symbologies. All of those are like ladder style 1D Barcode Symbologies.
2D Barcodes are normally read by using 2D barcode readers use camera technology to take a snapshot of the barcode, then process and interpret the symbology type, and translate the data. The data is typically delivered in the ASCII character range, ex: it translated(A, B, C and special characters), through a keyboard type interface, mainly USB Keyboard style. Devices using 2D can take a snap shot like a camera, if the software allows for that delivery capability. Obviously, encryption is used on some 2D devices where sensitive data is used and it is somewhat effective.
The advent of 2D Barcode symbologies opened up the ability to scan most 2D barcodes more reliably, even if the barcode is partially missing, since the barcode is typically repeated across different sections of the barcode and not just in a linear fashion, like UPS.
Additionally, some 2D barcode symbologies were better for laser etching , like Datamatrix engraving it directly onto parts. This etching on metal type items is called Direct Parts Marking. Some 2D barcodes like PDF 417, were idea for storing large amounts of data like drivers license, identifications information, and shipping data, especially large amounts of information that does not change. These are also seen along with a 1D barcode on Patient Wristbands. The 2D on wristbands helps in identifying specifics about a patient, even when the network or systems are not operational, since all data vital to the patient is printed in the PDF 417 2D barcode.
Normally the amount of data, multi-use capability(like DPM), durability, or another specific proprietary use, determines the barcode type or symbology used for 2D. QR barcodes have been a very popular 2D symbology the last few years, especially with companies using them for advertising. This has been used extensively by the smart phone industry, to allow for mobile access to web sites via the URL on the barcode using a quick scan instead of typing. Phone numbers can be dialed from scanning a QR, Information Address VCard Content, and even plain text can be imbedded in a QR code. Some QR allow for color logos, graphics, and even pictures, to be used in the barcode.