Generally, mod_ocaml is a flexible apache module for use of INRIA's powerful OCAML interpreter as a Web scripting language (with functional and imperative features). Simply embed OCAML code between HTML on your pages, like you do with PHP, mod_perl, etc. After modifying some settings, you can use almost any interpreter for providing dynamic content on your HTML pages.
mod_caml is a set of Objective CAML (OCaml) bindings for the Apache API. It allows you to run CGI scripts written in OCaml directly inside the Apache Web server. However, it is much much more than just that. You can bind to any part of the Apache request cycle, read and modify internal Apache structures, share modules of code between handlers and scripts, or use the CGI library and templating system (allows separation of code and presentation).
Ocsigen is a research project aiming at developing new programming techniques for the Web. It contains a full featured Web server and a programming framework that provides a way to create dynamic Web sites. Its goal is to offer an alternative to Apache/PHP, based on technologies from research in programming languages. There is also a module, called Eliom, for generating dynamic Web pages. With Eliom, you program in a concise and modular way, with a strong type system which helps you to produce valid XHTML. The server handles sessions, URLs, and page parameters automatically.