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.
SLIME is an integrated development environment for Common LISP which does everything you would expect from an IDE: code evaluation, compilation, macro expansion, and auto-completion. It also finds definitions of functions, and marks LISP forms which the compiler finds to be erroneous. It provides easy access to implementation-specific online documentation as well as the ability to look up symbols in the ANSI Common Lisp HyperSpec. Further, it includes an interactive debugger and object inspector.