It's the difference between being Lisp and being a member of the Lisp Family (aka "being a Lisp"). It's a fun way to weed out 1-bit people from the population: http://www.xach.com/naggum/articles/3225130472400367@naggum.... Furthermore Common Lisp is a Lisp-2 whereas Scheme is a Lisp-1. (Aesthetically I prefer Scheme.) So really there are several working meanings at play when people say "Lisp", this is frustrating for 1-bit people.

My usual heuristic in using context clues to determine which meaning is meant: if there is no qualifier for a particular family member of Lisp, I assume something is talking about the Lisp family, unless accompanied by a code example, in which case it's probably Common Lisp. When I see someone type "LISP" I usually think "This person doesn't know Lisp", just like how you'll sometimes see people type FORTH or PERL when talking about those languages. But that's not always the case so it's useful to double-check before calling them out, especially if they give code examples. (They may just be old after all. ;) )