Are there any differences between slander and libel?
Slander is spoken defamation, libel is written or published defamation.
Slander and libel are both forms of defamation, which is usually defined as a false or misleading statement, usually with the intent to harm somebody's reputation. Statements that are true or are genuinely believed to be true by their author would not usually be considered defamatory.
The difference is that the term "slander" refers to spoken defamation, while "libel" refers to written or otherwise recorded defamation. The law treats these differently for several reasons: written statements can last indefinitely and may be widely reproduced, greatly amplifying their power; written (and especially published) statements are usually more carefully planned out than spoken communication; and there is usually undeniable proof of written defamation (the document itself) whereas it can be much harder to prove slander.
Defamation laws vary widely by country (and in the US, by state) but usually the following factors are considered:
See also: Wikipedia's article on Defamation
Amusing sidebar: There have been several cases in which celebrities have sued small-time bloggers, or even anonymous users in online forums, for libel. Whether they win or not, media coverage of the event usually results in the potentially libelous statements getting much wider publicity than they would have otherwise, usually causing much more reputational damage than the statements themselves.