Tags are like keywords or category labels. Tags help you find content, events, and members which have something in common or similar interests. Tags can be added to groups, your profile, resources, wiki pages, and events.
When creating or editing content, your profile, groups, etc. you can add or remove tags as you wish. If a tag doesn't already exist, simply typing it in a "Tags" form field will create it and make it available for everyone to use.
You can't. Tags can only be deleted by the site administrators. You may only remove tags from a piece of content or profile.
When editing content, your profile, groups, etc. you can remove tags by simply deleting them from the list presented in the "Tags" form field.
No. You should be able to add as many tags as you want to taggable content/profiles.
No. Who tagged an item and what tags they added is currently not available to users.