This is a question I & my friends get asked so many times, I’ll clear it up here and now…

require
 The require() statement includes and evaluates the specific file.
 require() includes and evaluates a specific file. Detailed information on how this inclusion works is described in the documentation for include().
 require() and include() are identical in every way except how they handle failure. include() produces a Warning while require() results in a Fatal Error. In other words, don’t hesitate to use require() if you want a missing file to halt processing of the page. include() does not behave this way, the script will continue regardless. Be sure to have an appropriate include_path setting as well.

include
 The include() statement includes and evaluates the specified file.
 The documentation below also applies to require(). The two constructs are identical in every way except how they handle failure. include() produces a Warning while require() results in a Fatal Error. In other words, use require() if you want a missing file to halt processing of the page. include() does not behave this way, the script will continue regardless. Be sure to have an appropriate include_path setting as well.
 When a file is included, the code it contains inherits the variable scope of the line on which the include occurs. Any variables available at that line in the calling file will be available within the called file, from that point forward.

^From PHP.net^

In the probable event that the language above has not made anything about the two functions any clearer, the main difference is that the require() function will cause a fatal error to appear on your site where as include() will only give you a warning if it can’t find the file. I think it is clear now
Thanks.