Tags and Categories in Wordpress blog

Not long after WordPress.com began we started calling categories “tags” in some places, most notably our global tags system. wordpress.com/blog/2007/09/22/tags-and-categories So then, we select some categories and insert tags when write new post? Categories and Tags is same, or not? What is it blogger wanted ? If top portal website say that we start "tag - service" ... Yes I see! "Categories service" is web1.0, and "Tag service" is web2.0! Right? I don`t know...