In American jurisprudence, slander is considered oral defamation that is non-permanent, while libel is considered print/permanent defamation. Libel is usually more serious, because of its permanent nature. It now encompasses all forms of print, as well as video, etc. I believe internet publications would fall under libel laws, even if they are not as 'permanent' as printed material. I believe the same is true in other common-law-origin jurisdictions such as the UK and Australia, though I've not studied those areas of the law as extensively. One can be held liable for either libel or slander. Truth, however, is always a valid defense, even if the material is defamatory. It has to be false defamation in order to obtain legal liability attached to it.