The function of Apache can be enhanced by adding the module. First of all, there is "Core" that becomes the nucleus of Apache, and the module is added there and the function is enhanced. The module name is named in the custom with "mod_XXX". XXX is an outline name of the function. For instance, it becomes "mod_dir" "mod_alias" "mod_setenvif" etc.
The module can be added by "Static link" or "Dynamic link". A static link is a method to build the module into the execution file of Apache. Apache and the module integrate in the binary and, in a word, operate. A dynamic link is a method to make the module as another file, and to call the function from the file of the module if necessary. This function is called "DSO (Dynamic Shared Object = dynamic common object)". It is necessary to link "mod_so" modules statically beforehand to use the function of a dynamic link.
A dynamic link has the advantage in which the module can be put in and be removed only by reactivation though is the disadvantage that loads rise more than static links because of the call of the module function (The overhead hangs). It is necessary to compile the main body of Apache again to put the module and to remove though a static link can call the module function at high speed oppositely.