What is this classpath,path environment variables?

If you go to command prompt and write a command to run(or try to execute any .exe file) , OS will look for that file in current directory.. if it cannot find that...it will look for PATH environment variable... and search for that command in all the locations specified in PATH variable... a path variable might look like this

PATH=c:\windows\system32;c:\jdk\bin;

etc

Same concept goes for CLASSPATH variable...
when you run a java program.. the classes are looked for in the locations defined by CLASSPATH variable... if your interpreter cannot find those classes.. it will give you a class not found exception..


>>so is it common to all softwares,programs, and operating systems?
it depends how you define it.. if you define it as system variable.. then it is common for all programs/users... if it is a user variable... then it can vary from user to user.. you can also set the environment variable to have effect only on a particular session.. for example.. .in command prompt do the following

c:\>echo %PATH%
you will get the value of PATH

Now do this

c:\>set PATH=c:\bin

to see its changed value... write following

echo %PATH%

the result will be c:\bin... it will replace all previous value.. it is changed but only for this session.. now close this command prompt by writing exit..
start another command window
write
c:\echo %PATH%

you will see the previous value and the value you set is lost.. that means the value you cahnged was only effective with that session.. and its value in other sessions is intact



>>i mean do we use these variables common to all programming languages?
CLASSPATH is specific to java..

there are many other environemtn variables
for java you may have.. JAVA_HOME
for oracle you have ORACLE_HOME, TNS_ADMIN and many others

You can view the list of them by right clicking on My Computer->properties->Advanced->Environment Variables


Hope that helsp