Quick question, I was wondering who validates tags, is it the community or admins?

What do you mean by "validating tags"?

If you mean making sure that all of the post's tags are correct – the tags are crowd sourced, and anyone can edit tags on any post.
In case there is a disagreement / edit war, an admin usually steps in and locks the tags they feel are appropriate. You can request that an admin reviews the tags by reporting the post in question.

If you mean making new tags – those don't have to be approved. If you think that a new tag is worthwhile, you can create it and start using it. Make sure that there isn't a similar tag already in use, of course.
Of course, the community might disagree. The tag might be invalidated, or aliased to another tag here on the forums.

If you're talking a "valid tag" as in one that can be used on posts, anybody can create a tag by adding it to posts - without needing to consult anybody about it first. The tag is effectively valid until somebody proposes to invalidate it which generally requires community consensus before an admin will approve it. Admins can invalidate it without any consensus but generally don't.

Optionally if you're planning to make a new tag it's sometimes a good idea to make a post in the Do we have a tag for that? thread to make sure you're not going to waste your time making a tag that already exists.

The Tag Name Validator, of course!

Tag "validation" in this context only concerns whether new tags contain forbidden characters and/or sequences of characters. If you try to create a new tag using Unicode, for example, it will be automatically identified as invalid.