Enviroment variables in shell

In shell there are cretain important Environment variables , these vaibles can be seen by typing the following command : "env"
Some of the variables are PATH,SHELL,USER,MAIL. The PATH variable have list of directories as their values,these directories have the commands stored as shell script.we you type the command in shell, the shell looks into directories listed in PATH variable then it run the shell script ,so that the command is executed.SHELL variable have the shell assigned for the user as its value.Likewise the environment variable stores values which changes according to the user logged-in.