Code 39 is a widely used industrial barcode and is mandated for some automotive industry and Department of Defense labels, because of its ability to represent alphanumeric data.
Code 39 is defined in American National Standards Institute (ANSI) standard MG10.8M - 1983, and is also known as USD-3 and 3 of 9. It is discrete, self-checking, variable length symbology that can be readily be printed by a variety of technologies.

Every Code 39 character has five bars and four spaces, making a total of 9 elements. Of these nine elements, three are wide and six are narrow, making Code 39 a two-width code. Whether numeric or alphanumeric data is encoded, Code 39 requires the same amount of space - substantially longer than ITF or alphanumeric Code 128.
The length of a Code 39 symbol limits its use on corrugated board because the shipping container is often not large enough for the amount of space required for a readable code, or the symbol is too large for scanning equipment to read. Code 39 has historically been found on paper labels or on documents such as factory work orders, where smaller "X" dimensions are possible.

The Code 39 character set includes the digits 0-9, the letters A-Z (upper case only), and seven special characters (-.*$/+% and space).
A Code 39 symbol begins and ends with an asterisk (*), which is this symbology's start/stop code. It is not require a checksum, although a modulo 43 check digit may be appended for increased data integrity.

Each character is separated form its neighbor by a loosely toleranced intercharacter gap that contains no information. Because of the mirror image relationship between start/stop character and the letter "P," an upper limit is specified for the intercharacter gap width in order to prevent short reads, especially in the case of partial scans.

Although there are only 43 data characters in Code 39's character set, it is possible to encode all 128 ASCII characters using Code 39's Full ASCII feature. If a reader is in its Full ASCII mode, the symbol $, /, %, and + are used as precedence codes with the 26 letters.

Code 39 Extended

This is an extended version of Code 39 that supports the ASCII character set. So with Code 39 Extended you can also code the 26 lower letters (a-z) and the special characters you have on your keyboard.

When you see stars:
Code 39 uses stars as a start and stop digit. These stars (*) are normally not shown in the text line below the code. But sometimes you can find a Code 39 with the "start/stop star" in the text line; in order to create this code do not have to enter the stars during your input: If the text line shows *1234* you just have to enter 1234 because the stars are a fixed part of Code 39.