Vandalism is the conspicuous defacement or destruction of an unprotected article. Actions classed as vandalism include but are not limited to:

Articles

Adding unconstructive words/pictures/objects to an article.
Blanking content from an article.
Replacing a story/article with your own.
Spamming an article.
Replacing content in an article without author permission.
Replacing content in an article with something that violates the Fandom Terms of Use.
Adding spam categories to an article (e.g. "stupid story").
Adding photos or videos to someone else's story without author permission. This includes narrations.

Making edits to correct awkward phrasing, spelling, grammar, etc. IS NOT vandalism. Quite the opposite; it is helpful. Vandalism is destructive editing.

Heavily Vandalized Pages

After a page has been vandalized a minimum of five (5) times, it is to be protected for at least a temporary time.

If the vandalism continues, the page may be permanently protected.

User Pages

Do not edit information or other parts of another editor's User namespace page or talk page, unless the removed content is profane or inappropriate.
These rules also apply to articles.

User Talk Pages

Please do not remove any official warnings or ban notifications left by the admin team or Wikia Staff. Just because it has your name on it doesn’t make it your talk page. Blanking a message from your talk page can be considered vandalism. It’s meant to be a record, not your own public messaging system.
Changing the wording of someone else’s message is considered malicious.
You can archive your talk page using a subpage to your User Talk space, but you may not simply blank it. It can be a useable record in the future. You can contact an admin for assistance with this.
With this in mind, you may alter your own comments.

Forums

Removing replies that are legitimate to the post is considered vandalism, as it disrupts the flow of the discussion.
Removing in-session forum threads is considered vandalism.
Altering a quote in your own reply is not only vandalism, it’s stupid. The original reply will still be there. Correcting things like spelling is excusable, but changing the wording to take the quote out of context is not.

Administrative Response

If you vandalize, you will be blocked for:

2 weeks, first offense.
1 month, second offense.
2 months, third offense.
6 months, fourth offense.
Indefinitely, fifth offense.

Note that these times are dynamic and can depend strongly on the severity of the vandalism. Vandalism on User/Talk pages and forums ranges in severity, but should be considered less significant than that of an article. Intent should also be considered, and the administrator should fully investigate the situation before distributing a punishment they deem appropriate. Warnings should be issued in minor cases.