There are two kinds of site vandalism: intentional and unintentional. Both kinds of site vandalism need to be edited as soon as possible. It should be easy to determine what is a malicious form of vandalism. Swear words, hateful speech, etc. in an article's content warrant the banning of a registered user. However, if you see that a user has created a new page and typed in a nonsensical stream of letters, it is possible that they are just experimenting with the site's editing capabilities. You should edit this immediately and if it was made by a registered user, offer to help them if they are curious about the site's various tools. Also see Minimizing Vandalism.
Spam
Spam is usually defined as unsolicited advertising in the form of an email or forum post. Similarly, a wiki can become prey to spammers. You probably shouldn't be too strict with the addition of external links by users, as long as the link is relevant to your site's subject and is placed in a respectful manner. However, completely irrelevant advertising (especially those of the "adult" variety) should be removed immediately and using your own discretion, may warrant a ban. Also see the Spam page.
General Misuse
General misuse is any kind of improper editing/additions to the site. Like unintentional vandalism, it is often due to a person's ignorance of wiki technology. As a moderator, it is your job to help those users in need learn about what a wiki is and how to use it properly. Another problem that falls under general misuse is poorly written content, which also needs your immediate attention and editing skills.