When you try to execute a program in unix (one with the executable bit set), the operating system will look at the first few bytes of the file. These form the so-called "magic number", which can be used to decide the format of the program and how to execute it.

corresponds to the magic number 0x2321 (look it up in an ascii table). When the system sees that the magic number, it knows that it is dealing with a text script and reads until the next (there is a limit, but it escapes me atm). Having identified the interpreter (the first argument after the shebang) it will call the interpreter.

Other files also have magic numbers. Try looking at a bitmap (.BMP) file via and you will see the first two characters are . This magic number denotes that the file is indeed a bitmap.