In computing, a shebang or Hashbang is the sequence of character sequence consisting of number sign followed by exclamation mark (#!), when it occurs as the first two characters on the first line of the textfile. In this case, the program loader in Unix-like operating systems parses the rest of the first line as an interpreter directive and invokes the program specified after the character sequence with any command line options specified as parameters. The name of the file being executed is passed as the final argument.
For example, a file starting with the line:
#!usr/bin/python
Source : wikipedia