Common Lisp is a huge language, and Common Lisp systems tend to be massive and extremely complex. Emacs Lisp, by contrast, is rather minimalist in the choice of Lisp features it offers the programmer. As Emacs Lisp programmers have grown in number, and the applications they write have grown more ambitious, it has become clear that Emacs Lisp could benefit from many of the conveniences of Common Lisp.
The "CL" package adds a number of Common Lisp functions and control structures to Emacs Lisp. While not a 100% complete implementation of Common Lisp, "CL" adds enough functionality to make Emacs Lisp programming significantly more convenient.
*Please note:* the "CL" functions are not standard parts of the Emacs Lisp name space, so it is legitimate for users to define them with other, conflicting meanings. To avoid conflicting with those user activities, we have a policy that packages installed in Emacs must not load "CL" at run time. (It is ok for them to load "CL" at compile time only, with `eval-when-compile', and use the macros it provides.) If you are writing packages that you plan to distribute and invite widespread use for, you might want to observe the same rule.
