[Define what a Web application is]
[Often conflated with CGI.]
A delightful aid to work with Web forms is Don Libes' echoing Web page [L1 ]. Point a form at it, and it reports all the data it receives, in a format useful to developers. While not deep, it's handy.
The underlying reason is, of course, that Internet protocols are client-server interactions. To begin to program usefully, one must have two programs executing predictably--the client and the server. For many of us, maintenance of one program at a time is daunting. That's why it's such a blessing to have services like Don's, or the ftp-ftpd, smtp-smtpd, ... pairs in tcllib, or ... We ascend by securing our footing, one step at a time.