Glossary

Common Lisp:
Common Lisp, usually abbreviated as CL (and not clisp), is a language defined by an ANSI standard.
REPL:
read-eval-print-loop, an interpreter. Quite useful for interactive development.
Implementation:
A standard-conforming implementation of Common Lisp should be able to compile/interpret standard-conforming Common Lisp code.
SBCL and Clozure CL are some free implementations.

Resources

Practical Common Lisp:
Learn Common Lisp from the ground up. This is the resource that everyone recommends to people wanting to get started with Common Lisp. Asssumes basic programming knowledge.
Common Lisp: A Gentle Introduction to Symbolic Computation:
A good book for someone new programming.
Features of Common Lisp:
A tour of the various features of Common Lisp, with illustrative example code.
Sucessful Lisp
Another book introducing Common Lisp.
On Lisp:
On Lisp is probably the book with the best coverage on CL macros.
The Common Lisp HyperSpec:
The ANSI standard, in HTML.
CLiki:
The Common Lisp wiki.
Common Lisp.net
Hosts many Common Lisp projects.
Simplified Common Lisp Reference:
A simplified reference, listing the more frequently used symbols.

Implementations

SBCL:
A compiler that generates fast code.
Clozure CL:
A compiler preferred by OSX users.
ABCL:
An implementation of Common Lisp that runs on the JVM, and focuses on Java interoperability.
CMUCL:
A fast implementation that SBCL forked off from.
ECL:
A portable implementation that allows for easy interoperability with C.
GNU CLISP:
A portable implementation written in C.
LispWorks:
A commercial implementation which comes with an IDE.
Allegro CL:
Another commercial implementation.

Tools

QuickLisp
Quicklisp has, in recent times, become the de-facto method of obtaining open source Lisp libraries.
ASDF
Another System Definition Facility (ASDF) is a commonly used build system in the Lisp world. It is used behind the scenes by QuickLisp.
Slime
Superior Lisp Interaction Mode for Emacs (SLIME) is a widely used Emacs mode that turns Emacs into a Lisp IDE.
Slimv
Think SLIME, but for Vim users.