I am new person who is learning exchange server so that i want to know about front end & backend server purpose .why we need these to server set up in exchange server

For a small or medium sized business (say, less than 500 mailboxes), you probably do not need a front-end server. If you have more than one mailbox server, the front-end server gives your users a single place to go for OWA, POP3, IMAP4, ActiveSync, etc... You point the alias such as MAIL.ACME.COM to your front-end server and then the user does not have to know which mailbox server they are located on. You can also offload SSL to a front-end server so that the CPU load of the SSL encryption is handled by the front-end server. Originally, Microsoft suggested putting the front-end servers (if you use them) in your perimeter network, but that is generally a bad practice and most people do not do that anymore. The better recommendation is to "publish" your web resources using something like Microsoft's ISA Server. Is this helpful?

Thank you i am really thank ful of that you gave this important information