web based email

can someone give me a technical explanation on how they program the interface and what kind of stuff they use on the back end to make web based email services like yahoo run?

That’s an extremely complicated question. There are countless options.

Some things that matter are:
How you’re getting the mail to the server, i.e. SMTP, NFS from SMTP server, proprietary protocol, etc.

How you’re CGI will access the mail: i.e. POP3, IMAP, direct-file access, SQL database, some-other-thing

How you’re CGI will send new mail: i.e. direct-to-SMTP, spooled to mass-SMTP queue, etc

How you’re generating your dynamic HTML, i.e. Perl, PHP, Java Servlets, ASP (yuck), or your own program.

In short, every major web-based email thingie is probably rediculously different. There are a few free web-based mail packages available on freshmeat, mostly written in Perl and C and designed to connect directly to a POP3 server.