Tags are not case sensitive. A tag is defined the first time it is applied to content. Subsequent uses will revert to the same format as the initial time it was created.
For example, if I create a post and apply the tag "SharePoint" and this is the first time the tag has been applied it will be set to this format. If another member then creates another post and tags it "sharepoint" it will be tagged with "SharePoint"
Tags are also only displayed within the Group that they are applied within.