A series is a data structure much like a sequence, with similar kinds of operations. The difference is that in many situations, operations on series may be composed functionally and yet execute iteratively, without the need to construct intermediate series values explicitly. In this manner, series provide both the clarity of a functional programming style and the efficiency of an iterative programming style. Series is the culmination of many years of design and use of this approach, during which some 100,000 lines of application code have been written (by about half a dozen people over the course of seven years) using the series facility in nearly all iteration situations. This includes one large system (KBEmacs) of over 40,000 lines of code. In a nutshell: Think "Efficient MAPCAR". SERIES translates functional-style expressions into efficient loops.
ILISP is a package that is designed to integrate various Lisp implementations (mostly Common Lisp systems and various Scheme dialects, including Guile) within Emacs (or XEmacs). ILISP runs an inferior Lisp process (in Emacs parlance) and provides a specialized set of commands, key bindings, and menus to ease the interaction with it. ILISP commands access the underlying Lisp process and provide ways to make the editing, compilation, and execution of Lisp programs much easier.
CL-BibTeX is a replacement for the BibTeX program, written in Common Lisp. The aim is to enable the user to format bibliographic entries using Common Lisp programs, rather than using the stack language of BibTeX style files. A compiler that transforms BibTeX style files into comprehensible Common Lisp programs is included.
CL-PPCRE is a portable regular expression library for Common Lisp. It is compatible with Perl, and it's fast, portable (strictly ANSI-compliant), and thread-safe. It comes with convenient features like a SPLIT function, a couple of DO-like loop constructs, and a regex-based APROPOS feature similar to the one found in Emacs. In addition to specifying regular expressions as strings like in Perl, you can also use S-expressions which are more Lisp-y.
CL-Readline is an UFFI-based Common Lisp interface to the GNU Readline library. In addition to basic Readline functionality it features customizable completions, completions based on a Common Lisp package, multi-line expression reader, and support for non-echoing input. This software has been superseded by Linedit.