Barcodes started appearing on retail LP sleeves in the late 1970s, but adoption was slow in the industry; it wasn't until circa 1986 that major-label releases consistently had barcodes, and there were still exceptions after that.
The standard barcode formats for retail items are UPC-A and EAN-13.
UPC-A is a subset of EAN-13. Although UPC-A was originally designed to encode 12 digits and EAN-13 to encode 13, both formats effectively encode 13 digits, and nowadays share a common "EAN/UPC" definition.
The actual standards are complex, but the 13 digits represented by EAN/UPC can be thought of in this simplified way:
For the main 12 digits (the ones after the optional region code), each digit is directly represented by two pairs of bars and spaces of variable width. The choice of bar/space patterns used for encoding the first six digits imply the region code, which for UPC-A must always be 0 to indicate North America. A pair of extra-long guard bars always appears in the middle and on each end of the barcode.
A human-readable interpretation (HRI) version of the barcode is often printed below the bars, sometimes with the first and/or last digit separately to one side.
UPC-A and EAN-13 differ on how the HRI text is presented:
A barcode reader only sees the pattern of black and white bars. It does not read the HRI, and it does not check the length of the bars. Therefore, the scanner does not know whether it is reading UPC-A or EAN-13, unless it finds that the first six digits in the bars use one of the patterns that implies a non-zero region code.
A modern scanner normally interprets both types of barcode as a 13-digit string. If the first digit is zero, then depending on how the reader is configured, it may report the string as an EAN-13 with all 13 digits, or it may drop the initial zero and report the string as a 12-digit UPC-A. Free barcode-reading apps for smartphones and tablets often just report everything as EAN-13. In other words, when the first digit is zero, a barcode reader cannot be trusted to correctly identify the type of barcode; it could be UPC-A or EAN-13.
On Discogs, it is standard to not to mention the type of barcode at all when it is UPC-A or EAN-13, unless both types are on the same release (very rare). If you do wish to mention the type of barcode, do not just rely on the barcode reader; use your eyes to confirm the way the human-readable text is printed.