A bar code (also barcode) is an optical machine-readable representation of data.

Originally, bar codes represented data in the widths (lines) and the spacings of parallel lines and may be referred to as linear or 1D (1 dimensional) barcodes or symbologies. But they also come in patterns of squares, dots, hexagons and other geometric patterns within images termed 2D (2 dimensional) matrix codes or symbologies.
In spite of there being no bars, 2D systems are generally referred to as barcodes as well.

The first use of barcodes was to automate grocery checkout systems, a task where they have become almost universal today. Their use has spread to many other roles as well, tasks that are generically referred to as Auto ID Data Capture (AIDC).
Newer systems, like RFID, are attempting to make inroads in the AIDC market, but the simplicity, universality and low-cost of barcodes has limited the role of these newer systems.

It will cost about US$0.005 to implement a barcode compared to passive RFID which still costs about US$0.07 to US$0.30 per tag.

Barcodes can be read by optical scanners called barcode readers or scanned from an image by special software.
In Japan, most cell phones have built-in scanning software for 2D codes, and similar software is becoming available on smartphone platforms.