A charcter set is a set of characters that can be encoded using a given symbology. There are usually three types of character sets used by the barcodes: numeric, alpha-numeric and full-ascii.
A numeric character set includes numerals 0 through 9. A numeric symbology may include a few additional characters, such as start and stop characters that may be alphabets.
An alpha-numeric character set inlcudes numerals 0 through 9 and 26 alphabets of the English language.
A full-ascii character set includes all of the 128 ASCII characters. A full-ascii symbology may include a few additional control characters from the rang 128 through 255.
Each character in a continuous symbology starts with a bar and ends with a space. Since, the characters are placed adjacent to each other without any inter-character spacing, the first bar of a character terminates the final space of the preceding character. And the final space of the last character in the series is terminated with a termination bar.
A discrete symbology encodes each character in such a way that they can be interpreted individually without requiring the rest of the barcode. Each character in such symbology starts with a bar and ends with a bar and an extra inter-character spacing is introduced between the adjacent characters.
A fixed-length symbology requires applications to encode a certain fixed number of charcters. An application cannot encode any data with less than or more than the pre-defined length. For example, UPC-A is a 12-character symbology that always encodes 12 digits of data.
A self-checking symbology encodes each character in such a way that any single printing or scanning error does not result in the character being misinterpreted as another valid character.
A two-width symbology encodes data using bars and spaces that are either wide or narrow. This scheme provides a higher level of print tolerance, as once once it is determine how wide a "narrow" bar or space is, anything wider can be considered "wide."
A variable-length symbology can encode data of any length. There is no restriction on the number of characters to be encoded.