In the case of Wikipedia, as all modifications are authorized and none can be truly destructive, vandalism refers to undesired modification of its pages.
Since most modifications are desired and encouraged (see Wikipedia policy, Be bold in updating pages), only the most egregious cases of editing can be considered vandalism.
Even downright bad jokes and other nonsense are not generally considered vandalism, and often are simply cases of misunderstanding, confusion, or humor unnecessary to the entry.
Cases of Wikipedia vandalism involve the total replacement of on-topic content with off-topic content, often vulgar? or obscene, especially with the inclusion of off-server images.
The most inviting target for such vandalism is the HomePage. Its contents have been replaced at least ten times by anonymous people with off-topic material, which is why editing of the page is currently restricted to Wikipedians with administrator access. To gain access to editing the HomePage, e-mail Jimbo Wales.