Not really - one is a sub-category of the other. Defamation is lowering someone's reputation. It is divided into two sorts - in permanent form, eg a book, newspaper, film, etc it is called Libel; in temporary form it is called Slander, eg typically spoken at a gathering but not recorded. Slander is harder to prove and and damage from it is more likely to be temporary too, so courts are not inclined to treat it as a very serious issue. Libel can be repeated many times to very large audiences. Most defamation cases involve a libel rather than a slander so the terms are very much intertwined.
